Xray Equipment

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X- RAY EQUIPMENT FOR RADIOGRAPHERS

Fundamentals Of X-Ray Equipments

BASIC MANNER OF OPERATION OF X – RAY MACHINES THE BASIC CONTROL PANEL OF AN X-RAY MACHINE: Inspite of some variation in the present day automatic control panels of different x – ray machines, there is a basic design which can be recognized in all types.

The essential elements of a control panel are: 1) Main Switch 2) Line Voltage Meter 3) Line Voltage Compensator 4) Autotransformer 5) K.V. Meter 6) Timer circuit 7) Exposure switch 8) mAs selector Meter The main switch connects the x-ray x-ray circuits with the the power supply mains. The line voltage meter indicates the proper line voltage. The line voltage compensator adjusts the line voltage if necessary. The Autotransformer (A.T.F.) control selects the proper kilo-voltage to be applied for a particular examination. The voltmeter on the primary side of the step-up step-up transformer is calibrated to to indicate the KV being obtained on the secondary side of the transformer in the x-ray tube circuit. The timer circuit cuts off automatically the exposure at the present time. The exposure selector switch permits selection of Fluoroscopy or Radiography or spot-film / Bucky radiography. The mAs selector indicates the desired mA mA that is selected. This is wired in with an ammeter ammeter on the primary side of the filament filament circuit so that adjustments can be made if needed, by means of the filament control. It is possible to trace the action of the operator and the circuit in the following manner: 1) When operator turns the the switch on, the current passes in to the filament filament circuit and heats the filament, giving rise to flow of electrons around the filament. 2) Assuming that the A.T.F. is set the timer timer is is set, set, the exposure switch switch selected for hand switch radiography, if turned on 3) Current passes through the A.T..F. and through the primary step up transformer and then the the induced (oppositely (oppositely directed) current passes through the A.T.F. and through the primary step up transformer and then the induced (oppositely directed directed ) current passes through the secondary side of the step-up transformer , through the rectifier circuit and then to the x-ray tube. 4) The high P.D. is applied in this way between the tube tube filament and the target, forcing forcing the electrons from the heated filament to strike the target, thus producing x-rays.


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FUNDAMENTAL OF X-RAY EQUIPMENTS 3.1 THE ELECTRICAL SYSTEM AND MAINS SUPPLY TO X-RAY EQUIPMENT

A single single phase 3 wire mains supply will be sufficient sufficient for for an x-ray equipment of low low output such as portable sets or mobile units. This means a restriction of tube current not greater than 50mA and a tube voltage around 90 KVP as maximum. These equipments will draw currents upto 15 amperes. The wall mounted power sockets in wards, operation theatres or houses will be enough for the use of these small output equipments. Whereas x-ray units drawing tube currents from 200 to 500 mA and tube voltage upto 100 – 130 KVP need a three phase 4 wire mains-system. In these 3 phase systems, 2 phases are combined by producing line voltage of 415 volts are used for for tube and the remaining remaining one phase th supply is used for various circuits of control switches switches and bucky motors. The 4 wire in the 3 phase system is the earthing wire. All the 3 phases wires are combinedly used to feed the primary windings of the High Tension Transformer (H.T.T.) 3.2 CONTROL OF KV: Since the H.T.T. is of fixed turns ratio P.S. = 1:250 any change in the input line voltage in it, changes H.T.’s output voltage greatly (The KV of the tube supply) So the power supply from the mains is fed to the primary of H.T.T. through an variable input transformer called the VARIAC or STATIC VOLTAGE STABILISER or STATIC BALANCER. This VARIAC is an auto – transformer with one winding only which works on the principle of SELF INDUCTION. The AUTO TRANSFORMER (A.T.F.) can be used successfully to step up or step down the line voltage according to the needs, for controlling the KV in x-ray sets.

By selecting selecting the the number of turns turns in the the A.T.F. we can increase or decrease the input to the the H.T.T. and there there by the output voltage of the H.T.T. This is accomplished by the the use use of the STUD TERMINALS TERMINALS connected to the KV KV selector switch. These studs are used to connect a variable number of turns of A.T.F. with its secondary circuit by means means of a manual control switch called the KV SELECTOR on the the control panel of the x-ray set. Each stud stud position is marked on the KV Indicator disc with its KV value. The lowest being 40 KVP and the highest being 120 KVp. The KV indication is given by the KV meter on the control panel of the equipment when the exposure is made. When the mA selector is ganged (mechanically joined) with the KV selector to the A.T.F. coils, any change in the position position of the mA selector switch to get higher tube current, the KV compensator control ganged already is moved moved in that direction to select select more voltage for the primary windings of H.T.T. H.T.T . When the mA meter is moved to select lower tube current the KV compensator moves with it and goes to to select appropriate KV.


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~ 68 Handbook 68 Handbook of Medical Radiography

I U O G O T G N

~t J

F' XED XED CONTACT

U-SHAPED COPPER BLADE

AN AUTO TRANS FORMER AS CONTROL OF K. V.

AN AUTO

TRANSFORMER AS CONTROL OF K.V.

TO H.T RECT'FICATION AND x-RAY TUBE SUPPLY VOLTS FROM MAINS

=-0-'111 M.A. METER


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3.3


MAINS VOLTAGE COMPESNATOR (M.V.C.):

This is needed for supply of stabilized voltage for the “step-down” transformer of the tube filament (primary) circuit and also to other parts of the x-ray unit like the KV KV selection and other components in order to prevent damages due to sudden fall or increase in voltage. The M.V.C. maintains constant unchanged output from the A.T.F. despite the alterations in the line voltage due to fluctuations. If the voltage changes were to occur in A.T.F. that the output from A.T.F. will not give the correct KV from H.T.T. inspite of selecting the correct KV selection and lead to faults in radiography radiography M.V.C. can be achieved by manual control or by an automatic methods.

MANUALLY ADJUSTED MAINS VOLTAGE COM.NSATOR


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M.V.C. is needed when changes can occur in mains supply due to 1) slow variations in voltage occurring outside the x-ray exposure 2) rapid variation occurring both outside and during the exposure 3) a fall in voltage which is caused by the x-ray exposure occurring as soon as the exposure begins and lasts through-outs the duration of exposure. 3.4

THE COMPONENTS AND CONTROLS IN X-RAY CIRCUIT:

There are 3 important factors in production of a radiograph, 1) The penetrating power of the beam 2) the intensity of the beam 3) the length of the time for which the beam is directed to the film through through the patient. The penetrating penetrating power of beam is varied by changing the KV across the tube. Raising the KV accelerates the electrons across the x-ray tube towards the anode to make the x-rays more penetrating and vice- versa. The change in the intensity of the beam is given by the the alteration in the the number of electrons which are produced from the heated cathode filament. The hotter the filament, the more intense is the beam beam produced and vice versa. The electrons emitted by cathode filament form a current flowing through the tube during exposure. This current is called the M.A. Current.


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THE CONTROL UNIT


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The higher the mA current through the tube, the more intense is the beam emitted. The exposure time is the the mean length of time during which the x-ray tube is energized from the high KV source and the beam of the x-rays directed towards the film. This variable period is achieved by the use of a timer, which starts the switch switch of exposure and stops it also after the required duration. The main components of x-ray unit apart from the mains supply are: 1) the high Tension Tension circuit and KV control and 2) the filament circuit and mA control and 3) Timer 3.5 THE HIGH TENSION GENERATOR: The high tension voltage applied to the anode of the x-ray tube gives the kinetic energy for electron to leave the cathode and bombard the anode. For the diagnostic radiology, the range of voltage used is about 40 KVP to 120 KVP. These high voltage are obtained from a “step up” transformer which raises the mains voltage to the KV levels need for the working of x-ray tube. This “step up” transformer is called the High Tension Transformer Transformer or the H igh Tension Tension Generator (H.T.G.) OR THE Secondary Tube Circuit along with its other compoents such as rectifiers and relay etc.

To the H.T.T. the cables to the anode and cathode are connected by way of H.T. Switches at the top end of the H.T.G. Tank. The H.T.G. tank contains purified transformer oil which acts as both insulator and as a cooling agent. The tank is earthed to the ground. The parts of H.T.T. are as follows, the the core, the winding of primary coil, the winding of the secondary coils and their oil immersion. H.T. CABLES


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The core of H.T is rectangular in shape shape and is composed of number of laminated laminated thin thin sheets of iron alloys of nickel or silicon. These laminate are insulated from each other by painting them with varnish. The laminae are clamped together closely. The humming sound which we hear when the H.T.G. is energized is due to the repelling each other of core laminations when the magnetic flux between them is maximum. The purpose of lamination is to reduce the power loss through eddy currents which works the power through heat. The core of the H T is earthed. The winding of the primary coil is made of thick wire of lower resistance and a fewer turns of coils. This is because the primary coils carry high current but low voltage. On the other hand, the secondary coil is made of very many many turns of thin wire as it carries very high voltage (in KV) with low current current (1 amp) under high resistance. The core arm which carried the windings has a sleeve sleeve of insulating insulating material on which the primary coil is wound in layer after layer of turns, each layer being separated from each other by special insulated varnish paper until the whole of the primary coil winding is complete. The primary primary winding is earthed by a copper sleeve put on the primary winding. The windings of the secondary coil is done on an insulated tube covering the primary primary coil turns. The thin secondary coil windings are done layer after layer, insulating them by thin was coated paper. The potential difference between the beginning and the end of the coil is built up in stages through through each layer of winding. The secondary winding of H.T.T. is wound in 2 parts in order to reduce insulation material needed and also the space. The centre point between these 2 turns of coils is earthed, through the core. This is called the earthed centre point or grounded centre of H.T. Secondary winding.


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H.T. GENERATOR

INTERIOR VIEW


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The ratio between the primary winding turns and secondary winding turns is usually 1: 250 or 1: 400 The H.T.T. is oil immersed for purpose of insulation and cooling. Further the primary circuit (L.T.T.) and the rectifiers are also to be protected from from dust and moisture. Any air inside inside this tank will result in areing so so the oil is filled to the brim completely. The tank lid contains the anode and cathode terminals which are connected to the well insulated receptacle sockets and circuit breaking switches. HIGH TENSION SWITCHES FINE FOCUS

3.6 H.T.G. CIRCUIT: The simplest form of H.T.G. circuit consists of H.T.T. with x-ray tube connected directly to its secondary winding (negative end) is connected to to the cathode of the x-ray x-ray tube, the other end of H.T.T. H.T.T. secondary winding to the anode of x-ray tube. This is the form of H.T. circuit that is used in the portable or mobile x-ray equipment. This type of circuit is called the the self self rectified unit.


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SUPPLY VOLTS


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In a full wave rectified circuit the average and peak values of mA are much closer together. This means that, for the same reading on the mA meter, the mA comes to peak value which raises the temperature temperature of the filament, whereas the average value of mA determines the total radiographic output and the total heat output. This means that a higher mA can be used in full wave rectified unit circuits and shortest exposure times without overloading the tube. 3.7

CONSTANT POTENTIAL CIRCUIT:

A true constant potential circuit is provided for the x-ray tube by using certain devices in the secondary circuit of H.T such as a) capacitors b) electronic valves. A) THE CAPACITORS: are placed in the circuit across the output of H.T.T. and rectifying system. system. These capacitors (inside the control panel) function to change the pulsating wave form into rippling waveform. B) ELECTRONIC VALVES OR VACCUM TRIODE VALVES: which are placed between the x-ray tube and the H.T. output (in the control panel) after the capacitors, function to remove the ripple and supply the x-ray tube tube a constant ripple – free voltage which is selected and stabilized. These electronic valves are Triode valve type or Triode solid-state-type. They act as 1) a switch 2) act to absorb absorb the different different amount of voltage 3) the required effects can be brought about in in the valve by altering altering the grid voltage to the valve instantaneously 4) they are used to control the voltage. RIPPILING VOLTAGE FROM CAPACITORS


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3.8 THE FILAMENT CIRCUIT (PRIMARY CIRCUIT) AND CONTROL OF TUEB CURRENT THE FILAMENT TRANSFORMER: This is a double wound stepdown transformer with a ratio of 20.1 primary is to secondary, its its secondary winding supplying the filament with a heating current of 4 to 8 amperes at 10 to 12 volts. The primary winding of this transformer is connected to mains supply via the autotransformer which gives stable voltage. A resistor is connected in series between the auto-transformer and the primary winding of filament transformer. A movable control allows more or less resistance to the included in the circuit. Corresponding changes occur in the primary voltage. Thus filament heat is altered by controlling the current to the filament of the tube.

FILAMENT TRANSFORMER Fundamentals Of X-Ray Equipments

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resistor gives stepless control of tube current, 3.8.1 THE mAs SELECTOR: This adjustable resistor and allows allows mA to be altered while the x-ray tube is being energized. energized. It It provides free control tube-current within the limits set by the resistors. resistors. For radiographic exposures, the the control of tube current is achieved by a system called Pre-selection of tube current by resistance resistance control and it gives variation of mA in fixed stops by the help of a rotary rotary switch moved by the radiographer, over a series of marked mAs. In this circuit circuit valve. So each resistor gives different voltage drops across it. 3.8.2 VOLTAGE STABILISER: Is needed because the voltage for the filament transformer must be a stable one. A change in the filament voltage of about 5% leads to a similar change in the filament heating current and these changes results results in a change in the tube current mA which is greater proportion amounting to 22% to 30% which will be noticed noticed on the the resultant radiographs. In the absence of suitable compensators and stabilizers, changes in the main’s voltage cause the change in the filament voltage. Inspite Inspite of the M.V.C. included in the circuit of Auto – Transformer, the fluctuations do occur in the main’s voltage during the exposure. Then other types of devices are necessary to maintain a stable voltage on the filament transformer during the exposure and these devices are called stablisers. AUTO TRANSFORMER


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FILAMENT TRANSFORMER

These are of two types 1) Static 2) Electronic stabilizer. Both of them are automatic and instantaneous in action. A stabilizer is connected between the supply voltage and ‘mA”selector, in the primary circuit of filament transformer. The static stabilizers are called so because they have no moving parts. They are also known as choked condenser stabilizer which which indicates the important components in them. 3.8.3 FREQUENCY COMPENSATOR: Changes in the frequency of current causes an irregularity in the voltage voltage for the primary primary winding of the filament filament transformer (cycles / second frequency) This can lead to changes in x-ray tube current because they alter the filament voltage and filament heat. A 5% change in the frequency frequency can results results in a change in the the x-ray x-ray tube current which is noticeable in radiographic result therefore some correction for this frequency change must must be provided by devices called “frequency compensators” This can be manual type, to be adjusted by a radiographer or automatic frequency compensator.


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M.A. SELECTOR

3.8.4 SPACE CHARGE COMPENSATOR IN FILAMENT CIRCUIT: The modem x-ray tubes operating at high mA have heavy electronic emission from its filaments. These electrons tend to from a cloud around the filament called SPACE CHARGE. Not all electron is emitted from the filament are drawn across the x-ray tube by the anode voltage. A given setting of mA selector results in different currents through the x-ray tube according to the KV used for eg. One setting of mA selector switch could give 500 mA at 70 KVP, 450 mA at 50 KVP, and 550 mA at 90 KVP. A space charge compensator included in the circuit permits the same tube current over the whole range of current kilo kilo voltage when the mA selector switch is in a given position.

SPACE

CHARGE COMPESNATION IN FLAMENT CIRCUIT Fundamentals Of X-Ray Equipments

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The mA meter is a moving coil and hence cannot read AC. So rectification is provided provided to the meter to make this AC meter unidirectional, the mA meter reads the average value of mA. There are two scales for this meter, one for high range, i.e, to 500 mA for radiographic purpose and high, i.e., 0 to 5 ma for fluoroscopic purposes.

3.8.6 MAS METER: This meter scale of the control panel allows the radiographer to select the range of tube current and also the duration of exposure time, klas per the patients conditions and the nature of examination. For example in the 200 mA range, an exposure time time of 0.08 seconds gives 16 mAs required for the chest x-ray. The mAs meter is of a moving coil type. The current to be measured is passed through the coil of wire which is suspended in the magnetic field between the poles of magnets. The coil rotates depending upon the strength of the current and the time for which it flows. The final position of the product of current (mA (mA and time in /seconds). /seconds). So the meter reads in mAs. These scales of mAs are provide on the modern machine of control panel, of which one reads 0-50 mAs another 0-500 mAs etc., This scale connections achieved by means of shunt resistors interposed interposed between. The mAs meters and its connection to the grounded centre point of H.T.T.


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3.9. FUSES: The purpose of an fuse is to safeguard the electrical equipment form the harmful effects of an abnormally high current or voltage. voltage. Fuse is a metal metal resistor which melts due to heat produced while conducting conduc ting the abnormal current and so breaks the continuity c ontinuity of the circuit, and makes it an open circuit by blowing itself off. Fuses are rated according to the current that is permissible to be conducted through them, without burning themselves. Fuses can be in the form of a) simple wires of different thinness or thickness (5 amps, 15 amps and 30 amps) used for mains fuses b) in the form of fuse units which are of different size and shape. shape.

Basically a fuse unit consist consist of fuse element, fuse link and fuse contacts. Fuse element is made of ti, ti, tin-lead alloy, aluminum element in its container or carrier. Fuse links may be of many varieties but may be of a) simple glass cartridge with metal ends sealed to the internal wire, on the glass of which is printed the rating of the fuse b)ceramic cartridges carry a colour for identification and they do not enclose the fuse element inside but the fuse element element is outside. It has to be rewired when it blows off. Fuse contacts are either attached to or the are integral parts of a fuse link. link. The fuse contacts engage themselves with the fixed contacts carrier in the equipment, such as metal clips or porcelain, fuse bases. 3.10 Switches And Circuit Breakers: A switch is a convenient appliance which is used to close a circuit or open the circuit by putting it in ‘ON’ position and OFF position respectively. A circuit breaker also does the same job. The examples of switches in a x-ray unit are the main supply switch, the on / off switch for HT generator, the selector switches on the control panel and also the exposure switch.


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These switches are classified as 1) large manually operated ones for mains supply 2) magnetic relays used to operate one or more sets of contacts automatically to make or break electrical circuit 3) circuit breakers which use relays in protecting the electrical circuits from overload 4)the HT switch 5) micro –switches for different finer ope rations of the units. Every switch carries a moving limn or contact which get engaged or is engaged with the fixed contact in the fixed part of the switch when the circuit is closed or opened respectively. 3.10.1. MAINS SUPPLYSWITCHES :

situated on the wall of the x-ray room. Close to the generator (the (the control panel). The purpose of this is to enable all the parts of the x-ray unit to be switched OFFD D or ON from one point. These mains switches are usually of the type of a knife switch. The movement of the conducting or connecting arm closes or opens the circuit at the fixed switch contact. These switches can be of single pole or double pole variety or maybe triple pole ( as in the case of 3 phase mains switch). The connecting arms of ‘U’ shaped parallel copper blades mounted on a bar of insulating material which is rotated by means of a switch switch handle to ‘OFF; or ‘ON’ position. The whole system is enclosed in a strong metal enclosure which suitably earthed. 3.10.2. MAGNETIC RELAYS.

These are electromagnetic devices which function function as electrically operated switches. The essential components of a magnetic relay are 1) an electromagnet 2) a movable arm of iron which is mounted to the end of the armature whose opposite end is connected to a returning spring. When the coil the electromagnet, is energized, a magnetic filed is established. This attracts the armature towards the core of the electromagnet, which results in closure of the pair of o f contacts contac ts and so a complete c omplete circuit is established across, the terminals of the relay. Now the relay becomes ia closed switch. When the electromagnet is de-energiesd.

The contact of the armature gets disconnected from the contacts of the relay. Because of the returning spring being present at the other end of the armature, it jumps back to ‘off position’. The above described relay is normally open relay and there is also the other type which are normally closed relays. Relays are extremely useful little gadgets which in certain certain situations


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have maximum advantages over the manually operated switches. They make it possible for a low tension circuit to control another circuit which carries HT. 3.10.3.REED SWITCH :

This a special variety of electromagnetic relay it consists of a small glass vacuum tube 20mm. in length ands 2mm in diameter inside inside which a pair of normally open contacts are sealed. The extensions of these relay contacts are provided with lead of wire by are which this relay can be connected to the circuit. When the relay is is subjected to the magnetic field, the contacts come together and close the circuit and vice-versa.. these switches are connected usually to the undercouch tube or table tilting device to prevent accidents to tube and table, when being carelessly or mistakenly used in darkness during fluoroscopy. Read switches operate much more quickly than than magnetic relay. Their operating time ti 1.5 milli seconds (10 m/s for for small relay and 20 m/s for large relay ). Read switches are also also dust protected. 3.10.4. CIRCUIT BREAKERS: are of following types thermal relay Magnetic circuit also breakers.

A) THERMAL RELAYS: These are devices which protect the electrical equipment from overload, which use the heating effect of electric current to perform their function overload. This system is useful when the x-ray tube and H.T.G are enclose together in a single oil-filled chamber as in portable and mobile units. The thermal conductor relay often depends upon the metal strip used in them which expands when heated. The free end of bimetal strip strip bends when hot. The one end of the bimetal bimetal strip is fixed and the other end is free to bend and move. The strip is also capable of carrying the significant current. When the free end of the bimetal strip bends it opens up the circuit. circuit. This open thermal relay has to be manually reset to close the circuit by a triggering latch after the bimetal cools down.

B) MAGNETIC CIRCUIT BREAKERS: These are designed contractors of electromagnetic kind to prevent damage to equipment from electrical over load action and are susceptible susceptible for remote control like a thermal relay. Once tripped, magnetic magnetic circuit breakers are to bimanually reset for closing the circuit but can be reset immediately. The main ON/OFF switch switch of x-ray generators (control) often includes the magnetic circuit breaker for overload control.


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3.10.5 THE H.T . SWITCH :

Many times an x-ray unit comprises of more then one x-ray tube, sometimes two or three, connected to in a single HT generator. A switch is often necessary to bring one of the to use by selection. This switch switch is called the HT switch. It has no role role what is every in the exposure switching. Its function is mainly to connect the HT to one of the multiple x-ray tubes that is required. It must must carry high voltages when it is closed. So, it cannot be manually operated. It is immersed in the oil tank containing the HT controlled it is a very large sized electromagnetic switch.


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3.10.6 CONSTRUCTION OF H.T SWITCH The HT switch consists of: 1) A pair of receptacles or pots for each x-ray tube, 1 pot for each H.T cable i.e., one for for cathode cable, the other for the anode cable. 2) A contact or contacts at the lower end of each pot, 3) Two contacts which oppose the other set and are mounted one at each end of an insulated actuating bar (moving contacts of switch) 4) A large electromagnet which can attract a vertical limb of the actuating bar so that the moving contacts are held against the fixed ones. 5) Leads from the moving contacts for connection to HT and filament transformers. transformers. One switch like this is necessary for each x-ray tube to which generator is used to used to supply power. The HT tank contains a minimum minimum of two or often three switches side side by side. They are mounted above the transformers, the the cable parts being the upper most. The moving contract are normally held away from the fixed contacts by the force of gravity. 3.10.7 THE CONTRACTS AND LEADS:

The fixed contacts on the pots are of two types one has a plane conducting surface and the second bears a trio of circular prominence or projections. projections. The contact which has plane face provides connection to the single conductor lead (cable) from anode of the x-ray tube. The contact which has triple projections on the other hand provides connection to the leads (cable) from the cathode of the dual focus of the x-ray tube (broad, fine focus) In the HT switch anode contact carries single lead to to one end of the secondary secondary winding of H.T.T. the cathode contact carries single lead to one end of the secondary winding of the H.T.T. and also also to the secondary winding of the filament transformers. In this way the anode and the cathode are connected via a high tension cable and H.T. switch to the source of voltage. The x-ray tube is to be selected before hand for radiography concerned. The selected contacts make the connection inside the H T tank when the exposure is made. The metallic metallic contacts are usually made up of copper with a thin coat of silver on it some times as in in Biplane radiography (angiography) it is possible to make two x-ray tube expose simultaneously with a use of single H.T. generator during du ring which time the two H.T. switches are closed. 3.10.8 THE ELECTROMAGNET

The electromagnet moves the the actuating bar in a H.T. switch. It is a very strong one, to be able to pull the moving contacts upwards successfully against the force of gravity. It is the only part which is at low tension. From each eac h end of magnet’s large coil, a lead lea d is taken to the top of generator and then to a rotary selector switch on the control panel (or a series of press button switches). This switch is marked to indicate the appropriate x-ray tube selected and allows the energizing voltage to be put across a corresponding electromagnet in the H.T. switch magnet tank. When the selection of the tube is made the armature contact of the switch an be heard Fundamentals Of X-Ray Equipments

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making sounds. This switch also combines the functions other than selector of x-ray selection of x-ray focus, selection of tube mA. 3.11

tube

INTERLOCKING CIRCUITS: (I.L.C.):

This terms means the the devices and circuits which are intended to save the x-ray tube by preventing an exposure being made which will result in over heating of the target and lead to its permanent injury. The following are examples. 1) I.L.C. in tube stator circuit 2) Delay circuit of tube stator 3) Overload interlocks 4) Mechanical interlocks 3.11.1 INTERLOCK IN THE TUBE STATOR CIRCUIT

A radiographic exposure made made while the anode is stationary which should have been rotating on the other hand, will severely damage da mage the x-ray tube. A simple interlocking circuit will not allow such accidents to occur (if the anode is not rotating due to to any fault in its stator stator circuit) The exposure contactor coil must be energized to obtain the radiographic exposure. ‘P’ and ‘E’ are 2 customary positions. Preparations and exposure stations, in the exposure switch. When the switch is in a station ‘P’ it initiates the rotation of anode. When the switch is in ‘E’ ‘E’ station, then the exposure is allowed allowed to occur. When ‘P’ is operated, it allows circuits of contact coil to be completed at E, as the radiographer continues his pressure pressure on the exposure switch. However, if there was a defect in stator winding, winding, which results in in an open circuit condition, the appropriate relay coil is not energized. Thus the radiographer cannot get radiographic exposure if the anode rotation is not functioning.

3.11.2 DELAY CIRCUIT WITH TUBE STATIOR:

The standard rate of rotation of anode of an x-ray tube is about 3500 RPM, and an interval of 0.8 sec is needed for it to reach this speed. Radiographer must be aware of the need to allow time for the rotating anode to reach its correct running speed. So, they must hold the switch in “prepare” position long enough to permit this speed of rotating anode to be attained. In situations like radiography of a restless uncooperative patient, he under pressure, might easily use the exposure switch prematurely and harm the tube. To avoid this harm from occurring, a DELAY CIRCUIT is introduced between the ‘prepare and’ expose positions so that even if the operator goes through straight from one to another, the exposures begin only after a delay of 0.8 Sec. when the rotating rotating anode reaches its proper speed. This delay circuit consists consists of a capacitor resistance resistance connected in parallel with relay coil.


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3.11.3 OVERLOAD INTERLOCKS:

X-Ray tubes may be over loaded loaded if selected exposure factors are too length. length. So x-ray generators normally include interlocking circuits which prevent radiographic exposure at wrong exposure factor setting. There are several ways in which such an overload interlock may operate. Presently electronic methods are in use. Some times mechanical interlock systems may also be found to be in use, in old machines.

AUTO TRANSFORMER

a) ELECTRONIC INTERLOCK (ANALOGUE) CIRCUIT: Is one which performs electronic addition sum. In effect, this circuit adds together the voltage which are the representatives of each one of the factors comprising the tube load (KV mA and Exp. Time in seconds. This form of a overload interlock allows the radiographer a free free choice of KV, KV, mA and S independently but at the same time does not exposure when too high KV and too high MAS are together wrongly selected by him because the aggregate analogue voltage will be too high to result in ‘open switch’ position of the transistor. Similar is the case when he selects too low KV and mAs below a certain level when again the transistor remains open due to lack of voltage across it. b) MECHANICAL INTERLOCK: The method of overload control, is present in mobile x-ray equipment. A purely mechanical mechanical and simple device operates on the principle of physical obstruction of controls in certain positions. The arrangements of interlocking depends on a chain. A lever and a cam. A cam is a wheel of eccentric design which revolves to Fundamentals Of X-Ray Equipments

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produce an alternating rectilinear movements. When cam is turned in a clockwise direction to a certain position, it results in a linear excursion of a pivoted bar. The mA selector and KV selector are connected together by a chain which passes also round a pulley at the free and of a pivoted bar level which is pring loaded loaded in order to hold the chain taut. RACHET


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In this situation prescribed, if the interlock is at overload point which is fixed i.e. At 10 KW, which means that the maximum permissible load might be 100 MA MA at 100 KVP or 200 MA at 50 KVP. Selection of factor more than the above range will not permit the timer cam to pull the lever upwards beyond the permitted levels. If the operator insists on selecting a long exposure time then the combined action of the lever and chain will together turn the KV control anticlockwise to a lower KV selection. The appropriate rating chart must be consulted whenever necessary, in order to prevent tube over load.

3.12

EXPOSURE SWITCHES AND EXPOSURE TIMERS:

In case of exposure there are 3 elements to be considered 1) Exposure must be started 2) It must be timed 3) It must be stopped

EXPOSURE CONTACTOR COIL

For precise control, the timing timing must begin as soon as the exposure process starts starts and must stop stop as soon as the selected period of time has elapsed. Precisely it is the flow of the tube current in the circuit of H.T.T. through the x-ray tube that is to be started and stopped. For this a switching system is is needed which can include a timing timing system in it. it. It must be capable of shorter exposure times and long exposure times times and must operate without the errors of starting late or by causing the exposure too longer than selected. In In small portable sets less than 0.5 sec exp is not possible. In high power machines, we can small portable sets less than than 0.5 sec exp is not possible. In high power machines, we can get as small small time time as 0.01 sec. Exposure switches may be any of the following tubes: Fundamentals Of X-Ray Equipments

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1) 2) 3) 4)

Mechanical contactors Mechanical contactors controlled electronically Electronic Primary Switching Switching in the secondary H.T. Circuit.

3.12.1 MECHANICAL CONTACTORS:

When the Radiographer presses the exposure button, the circuit for for the coil S is completed through the timing system system and contacts C are closed. closed. This completes the primary circuit of the H. T. T. Current flows in the primary and secondary circuits and the exposure begins. When the timer timer has completed the selected exposure intervals, the current through the coil “S” is stopped through the timing timing system. system. This causes the contact C to open, breaking the primary circuit of H.T.T. and thus stopping the exposure. RETURN SPING


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These mechanical contacts must must have certain properties: 1) It must be spring loaded so that they close firmly and without bounce. The spring must be strong enough to k eep the contact pieces together toge ther or be able to pull the contact pieces together

or be able to pull the contact pieces apart to open the circuit quickly and

consistently. 2) Since the contactors operate through moving parts there

will be some inertia or delay

associated with with it. it. These periods of delay must be as small as small as possible. 3)

The copper contacts which close close and open the circuit must be strong enough not to get distorted by continuous (exposure) closure.

4) The contact pieces must be able to with stand high high temperatures form arcing . so, they they must be made of Tungsten alloy.

Alternatively, the arcing may be reduced by

connecting 2 contactors in parallel with each other, both being in series with the primary winding of the H.T.T.

3.12.2 ELECTRONICALLY CONTROLLED MECHANICAL CONTACTORS:

To operate the mechanical contactors quickly, both to initiate and to terminate in a short time, a specially designed method is needed. For this purpose, 2 separate exposure contactors are used. One initiates and the other terminates the exposure. The coils of the 2 contactors may be controlled by separate initiating and terminating electronic circuits. One of these closes the primary circuit of the H.T.T. at the beginning of the exposure. expo sure. The other opens the circuit at the end of exposure. The contactors which opens the circuit is called the the BREAK contactor. The “MAKE”

contactor is normally open and when its coil is energized by current, it closes. The

“BREAK” contactor is normally closed and when its coil is energized by current, it opens.


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PRIMARY WINDING OF H.T. TRANSFORMER

It is possible to to arrange for the primary circuit to to close and open at zero voltage points in the the cycle of A.C. mains supply to make the radiographic results consistent when very short exposures are used. It is called phased switching.

3.12.3 THYRATRON

Valves in phased switching:- A thyratron is an electronic switch which can be opened or closed very easily first by altering electrical characteristics in circuits where it is used . It becomes conductive instantly at a certain grid voltage. The Thyratron is a voltage sensitive device with out any time delay. They thyratron has 3 electrodes - a cathode, an anode, and a pepper pot type of metal cylinder surrounding surrounding the cathode called the “GRID”. “GRID”.

The voltage on this grid can be used to control the operation of the Thyratron, because when the voltage on the control grid is sufficiently negative with respect to the cathode, no electrons can pass through the metallic grid and the thyratron is noncon ductive. So it becomes an open switch. If the negative voltage (called the negative negative bias) on grid is reduced, a point is reached at which the stopping power of the grid suddenly ceases and the valve becomes instantly, conductive - it becomes a closed circuit.


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3.12.4 ELECTRONIC PRIMARY SWITCHING

A) THYRISTORS: These are semiconductor solid state equivalents of gas filled Triode valves or Thyratrons. These are called THYRISTORS or SILICON CONTROLLED RECTIFIERS, and in one method they are used used as the electronic switches. switches. There are small and large Thyratrons. The large ones being suitable for switching big currents - 250 amps at 500 Volts. Such Thristors can be used as switches in primary circuits in 1000 mA high KV x-ray generators. These Thyristors are so small that they must be protected from overload with the help of rapidly acting fuses. B) TRANSISTORS: These are also semiconductors equivalents if vacuum triode valves. They cannot be used to switch switch currents at high voltages of x-ray tube circuit. Transistors Transistors are widely used in various other places in x-ray circuit - for the operation of closed circuit TV associated with image intensifiers and in control circuits of modern x-ray units. A transistor element is of one square millimeters millimeters and is in a container of 1 cm x 0.5 cm. size Even smaller transistors are used in the integrated circuits which is by itself a collection of many transistors on a long crystal of SILICON. SILICON.

3.12.5 SWITCHING IN SECONDARY H.T. CIRCUIT:

Exposure devices inserted directly in the H.T circuit must be wholly electronic and also of vacuum type. This is done by putting special triode valves into the the circuit of x-ray tube. These triodes are particularly designed to act as electronic switches for circuits circuits operating at high KV (say upto KVP) and they directly make and break the tube current (mA through the tube). These triode valves can be used used in various ways by altering voltages on their their control grids of the Thyratrons. The Thyratrons can be do more than just act as switches opened or closed. They can be used to absorb different values of voltage by alteration of their grid voltage. Thus they can be used in H.T. circuits in which they have more than one function to perform i.e. they switch the circuit and may act also to stabilize stabilize KV. Triode valves are connected directly directly into x-ray tube circuit. The triode valves have their own control circuits for alteration in their grid voltages according to the functions which they are intended to perform. Triodes acting as switches in H.T. circuit may be used in 12 Pulse 3 Phase generators. Very short exposures become possible with such arrangements, as short as a fraction of a millisecond. 3.13

TIMER SYSTEMS.


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These timing devices range from simple mechanical timer timer to the most sophisticated photo timers. In between are the electronic timer and ionization timer. The complex devices give very accurate timings and are used in high power machines and the less complex devices are used in low power machines, where high degree of accuracy is not that essential as the radiographer does not select less than 0.25 or 0.5 sec. On the other hand, in high power machines such as 1000 mA machine will need accuracy of 0.01 sec. An error of 0.01 sec in 1000 mA mA machine will result in 100% error in resultant radiograph. 3.13.1 MECHANICAL TIMERS:

Small portable and dental sets use clock work hand timers, in which a spring provides the motive power for for the timer movement. The hand timer has a scale marked marked on its dial with timer intervals form 0 to 10 seconds, and each second divided in to 4 equal divisions. The time is selected by moving the pointer over the scale to the desired value the exposure is made by depressing the button on the top of the hand timer this initiates the the exposure and the pointer being its return journey to the zero position. This timer is properly earthed and has a long cable connecting it with the primary primary filament circuit. The timer timer cable must be as long as 3 to 4 meters, to be able to allow the operator to go as for away from the equipment as possible to prevent unnecessary scattered radiation reaching him.

3.13.2 ELECTRONIC TIMER:

The basic principle in an electronic timer is the charging and discharging of a capacitor which actually initiates and terminates the exposure respectively and thus determines determines the duration of exposure. In order to make a capacitor stop the exposure at the end of a certain time, a voltage sensitive device must be included in the timer circuit which will act as a switch to open or close the circuit. This device can be a Thyratron or a Thyristor. When the x-ray unit is switched on, voltage from the 10V tapping on the transformer is fed via a full wave rectifier through the contacts (D) which are normally closed to the capacitor (E) The capacitor thus becomes charged.


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SIMPLE ELECTRONIC TIMER

The 240 V tapping from from the transformer is fed via a full wave rectifier rectifier to the Thyratron Thyratron anode and cathode. In series series with anode lead is exposure contact coil which operates the exposure contactor. Thyratron acts as open switch switch for this circuit. circuit. It It becomes a closed switch only when the exposure contactor is closed due to the current reaching it. When the exposure button is is depressed, depressed, contact (C ) operates. It It opens from (D) and closes to ©. This disconnects the charged capacitor (E) from the (F) which then comes to possess its striking voltage and becomes conductive. It It is now a closed switch. The exposure contactor coil is energized by the the current flowing in the anode circuit of the Thyratron and the the exposure contractors closes. The exposure begins now. During the exposure, the capacitor (E) discharges through the the time time selector and the the length of time discharge takes depends on the value of selected resistance (Time in seconds or fraction of it). When the capacitor discharge current becomes low enough, the Thyratron ceases to pass the current when the voltage next is zero. The supply to the the exposure contactor is interrupted due to the reason that Thyratron Thyratron has become open switch. Now the exposure contactors open and the exposure stops.


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PRIMARY WINDING OF H T TRANSFORMER

In order to time time these short intervals with greater accuracy, it is required to arrange 2 Thyratrons in the circuit. One of these initiates and the other terminates terminates the exposure. Similar arrangements can be made by use of Thyristors instead of Thyratrons, where the voltage is fed to the Thyristors gate (instead of thyratron grid) to make the Thyristor a closed switch.

3.13.3 AUTOMATIC TIMERS:

Automatic timers terminates the exposure after a required required amount of radiation has reached reached the film via the patient. Concerning this is the most important factor of absorption by the patient’s body under radiography. An automatic timer overcomes these difficulties of varying absorption in the patients because the exposure is is terminated only when when film film has received the dose of radiation necessary to to give it the required photographic density (after processing). Thus all radiographs are correctly exposed. Automatic Automatic timers were first introduced in MMR but they are now increasingly used in all a ll x-ray departments. Fundamentals Of X-Ray Equipments

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All Automatic timers employ the same principles but they are of 2 main categories 1) photo timers using photo-electronic current 2) Ionisation timers using ionization current. But these types use the same same principle of production of an electric current which which has the magnitude proportional to the dose rate of x-ray film.

1) PHOTO- ELECTRIC TIMERS (PHOTO-TIMER): Basic of an automatic timer which uses a photo electric current is the PHOTO ELECTRIC CELL. A photo electric cell has a glass envelope and it contains a) an anode in the form of a single stout vertically mounted wire b) the cathode is in the form of a reflector. The anode is mounted mounted vertically at the central axis of the curved cathode. The curved cathode is made of metal foil foil coated with caesium or compound of caesium and antimony, which has the ability to emit emit electrons under the action of light. This cell is connected into 2 simple simple continuous circuit with a voltage applied to the cell, its photo emissible cathode being connected to the negative side of the supply.

PHOTO TIMER


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An useful useful fact fact of P.E.C. or P.M.T. is that its strength is directly dependent on the intensity of light reaching the cell. The greater is the light reaching it the more is the current passing through the circuit of P.E.C. if the cell is put into a light tight tight container, one side of which has in it a fluorescent screen which can be activated by x-rays , then this arrangement allows the current flowing in the cell circuit, proportional to the intensity of radiation reaching the screen which is virtually the same as that reaching reach ing the film.

If this P.E.C. at the back of and below the x-ray cassette in a light tight radiolucent container - it is inactive, when the x-ray tube is idle. But when the x-ray tube is energized and is producing x – rays the radiation passes through the patient p atient to the film and then through the film to the fluorescent screen of P.E.C. which emits light as a result. This causes P.E.C. to pass current though the associated circuit.

The intensity of radiation to the cassette ( and so to the P.E.C) depends on 1) Tube mA 2) tube 3) patient absorption. An increase of mA and KV and decrease in patient absorption increases the radiation reaching the cassette or P.E.C.A decrease in mA or KV and increased patient absorption results in decrease in radiation reaching the P.E.C. So we have an arrangement by which the strength of a small current varies with the intensity of radiation reaching the film.

2) IONIZATION TIMER: In the present day diagnostic radiography these ionization timers are increasingly used. Here the x-radiation passes through a Ionization chamber when the x-rays ionize the air contained in the chamber if a potential difference is maintained between the electrodes of this ionization chamber, by connecting them to a source of voltage, in a circuit then the magnitude of current causing ionization occurring in the air of the chamber will be dependent on the intensity of radiation reaching the ionisation chamber.


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This ionization chamber can be placed in the position, between the patient and above the cassette, as it is very thin. As with with a photoelectric cell the ionization current cell be fed to a capacitor and used to charge it, it, the magnitude of the current determining the length of time which this takes. The voltage rise in the capacitor can be multiplied by special techniques and used so that when it reaches a certain value, a Thyratron or a Thyristor becomes conductive and the exposure stops. 3.14

THE X-RAY TUBE:

An x-ray tube is an electronic device like a diode for production of x-rays. The simple x-ray tube is a sealed glass envelope or bulb with a vacuum and 2 parts parts known as the ANODE and CATHODE facing each the and are mounted on their respective supporting arms arms which are electrically connected to the circuit outside the glass tube. Various types of tubes are there 1) Gas tube or crokes tube 2) the hot-filament tube or Coolidge tube. Among Among these, the former former is not in use now. The Coolidge hot filament tubes are again of different types. These are a ) Universal tube b) Metalix tube c ) Rotating anode tube d ) Low voltage tube.

BASIC FEATURES OF THE THE MODERN X –RAY TUBE

It has a sealed glass tube with complete vacuum of the orders of 1/10,000 mm mm of mercury which enables the tube to prevent any electrical discharge. In to this glass tube are sealed 2 arms, cathode at one end and the anode at the other end facing each other but separated by a short distance from each other. The cathode carries a filament of Tungsten spiral about 1/8” Fundamentals Of X-Ray Equipments

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diameter in a cup shaped holder. This filament is heated by a separate filament transformer to produce electrons. The temperature of the cathode filament dictates the number of electrons released. 0

The filament filament is heated upto 2000 C rapidly for this purpose. This filament is shaped so as to focus the electrons towards the focal spot on the anode target. The anode target is a small block of ½ square in the face of the anode which is supported by a copper arm. The other end of this this copper arm projects outside the glass tube for electrical connection with high KV circuit with the applying of high KV to anode, the electrons produced at the cathode are attracted towards the anode target. These electrons bombard the target, when the x-rays are p[roduced along with enormous quantity of heat. The process of heated Tungsten wire emitting electrons as a result of heat is called THERMIONIC EMISSION. The features associated with it ate: 1) The temperature and surface surface area of cathode filament which increases in direct direct proportion with increase of the above. 2) The KV applied to the anode which is positively charged 3) A change in the voltage across the tube changes the current through it and the two cannot be changed independently. 4) Eventually all electrons emitted are being collected and no increase in voltage voltage can bring any more across (saturation). At the saturated levels the current and voltage can be charged independently of each other. Equipment for diagnostic radiography involves two types of design of x-ray tubes. These are the FIXED ANODE x-ray tube and the ROTATING anode x-ray tube.

3.15

THE FIXED ANODE X-RAY TUBE:

The following parts are present in fixed anode x-ray tube 1) Cathode 2) Anode 3) The glass envelope and vacuum 4) The tube shield 5) Cooling mechanism 6) Filtration mechanism.


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3.15.1 THE CATHODE:

The cathode in the negative pole of the x – ray tube. It is metal structure supporting the filament which on heating emits electrons. It is designed to focus the electron beam leaving the filament. FILAMENT: is made of Tungsten wire which tolerates very high temperature 0 upto 3000 C . It is in the focus of helical spiral winding, so has to have large surface area for emitting electrons. It is of high resistance so as to to produce maximum heat. Its size is small, small, so as to produce electron beam covering a small area. These electrons emitted by filament filament are negatively charged particles having tendency to spread away from each other, so some mechanism is necessary to attract these electrons to force a beam, of small area. So that the electrons will bombard a small area on the anode to to produce x – rays that can give sharp radiographic images.

FOCUSING OF THE ELECTRON BEAM: The electrons that are spreading away from the filament filament are brought together by means of the electric field field which exist between anode and cathode. The filament sits in a slot in the cathode structure. The filament filament and slot are carefully designed in size, shape, and position, so that electrons leave the filament through the slot by this focusing the electrons come together in a beam so as to cover small area on the anode, called the FOCAL SPOT of the x-ray tube.


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The size of the Focal spot spot is determined determined by: a) b) c) d) e)

The Size and shape of the filament Dimensions of the focusing slot and the depth de pth of filament in the slot Characteristics of the electric field associating with the focusing slot The spacing of the the two electrodes and the distance between each other 2 2 The focal spot is of the shape of rectangle of the area 3mm to 15mm


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3.15.2. THE ANODE

This is essentially a metal plate to receive the electrons which bombard it. It is specially designed, so so that that the the electron electron bombardment bombardment gives rise to great amount of heat and a small small amount of x-ray x-ray radiation, and such that that the x-rays produced give radiographic images which are sharp in outline. Dissipation of heat is very important important to prevent tube damage and this dissipation of heat is provided by spreading spreading the heat anode of over a large area.


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In order to withstand withstand the heat produced, the anode must be made of metal which has a higher melting point. It must must have a high atomic number and be efficient to produce x-rays. It must be a good conductor of heat. It must not vaporize easily (as the vapours spoil the vacuum tube) It can be worked and made smooth surfaced. Tungsten provides the answer with all the the above features to be metal of choice for the anode target.

A piece of Tungsten in the form of a plate 2mm thick, rectangular or circular in shape and larger than the focal area is embedded on a thick copper rod which is a massive cylindrical block together form the anode. The Copper rod which is a good conductor of heat accepts the large amount of heat that is produced at the target due to electronic bombardment and dissipates the the heat accepts the large amount of heat that is produced at the target due to electronic bombardment and an d dissipates the heat h eat to the outside of the tube. tube . The outer end of the copper rod also serves as the electrode to which the positive positive pole of H.T supply is connected. The anode of the x-ray tube has a sloping face. This slope allows x-rays produced at the focal spot leave the tube sideways sideways and the radiographer uses type x-ray beam which is emitted emitted about an axis at right angles to the long axis of the tube. The middle part of the beam is the central ray which is perpendicular to the long axis of the tube and is surrounded by the divergent rays. The sloping anode is able to serve serve the 2 demands - a large large area to take take the electron bombardment and the small small x-ray producing target focus.


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The x-ray tube with large actual focus can be operated at higher mA without melting the target. Higher mA allows shorter times of exposures to be used and hence a large actual focus is preferred on the other hand, smaller actual focus produces sharper radiographic images.

The sleeper the angle of slope of the anode face the smaller is the apparent focus for a given size of actual focus. The anode slope must must not be 1) too steep to prevent the useful useful beam to become unuseful and narrow and 2) not be less steep enough to foreshorten the actual focus adequately so as to render the the resulting resulting images unsharp. In In the the modern diagnostic tubes, the anode angle varies from 7 to 20 degrees.


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Some examples of field sizes covered in relation to different anode angles and tube film distances are given.

TARGET ANGLE 7

0

TUBE FILM DISTANCE IN CMS 100

FIELD SIZE OF FILM COVERED

25 X 25 CM

10

0

100

35 X 35 CM

15

0

100

53 X 53 CM

20

0

100

73 X 73 CM

3.16 THE DUAL FOCUS X – RAY TUBE:

Radiography of thin parts of body like extremities extremities of a cooperative patient may be done with small focal area which allows only lower mA and long exposure times. times. But for for thicker thicker parts of the body and regions which contain organs with involuntary movements, a large exposure dose and at the shortest exposure time is used to reduce the movement blur.


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In these cases, we need a larger focal area which allows the use of higher mA and short short exposure time without damage to the target. So, we need tubes with different sizes of focal spots according to the subjects to be examined. Hence the incorporation incorporation of a dual focus focus in a single x-ray tube. The larger focus is known as the Broad focus and the smaller smaller one is known as Fine Focus. Modern x-ray tubes are all of the dual focus type except in the portable unit.

Dual focus x-ray tubes have 2 filaments filaments mounted usually side by side side on the cathode structure, each filament sitting in its own separate focusing slot. The broad focus filament is bigger than the fine focus filament. i.e. it is helix of a greater length and it is in a longer and wider slot. On the anode the Tungsten target accommodates the 2 different focal areas superimposed on each other. There is a margin of Tungsten allowed round the focal areas. So that the heat may spread from the place of its generation on the target to the copper rod. Each of the filament is heated by a transformer winding, when the radiographer selects the focus, the appropriate filament filament is heated by respective transformer energizing. The circuits are so arranged that it is not possible to heat both the filament the same time.

3.17 THE GLASS ENVELOPE AND THE VACUUM:

The enclosing glass envelope serves to support the anode and cathode and maintain the vacuum. The envelope must have the following characteristics: 1) It should be able to with stand heat and mechanical stress; 2) It should be an electrical electrical insulator able to with with stand high voltage; 3) It should be capable of being sealed to the electrodes with a vacuum tight heat proof seal.

The substance chosen for modern modern x-ray tube is a hard heat resistant glass. This glass is not of uniform thickness throughout. throughout. The glass is thinner at the window where the useful x-ray beam emerges, the prevent prevent any change in the intensity or quality. The glass envelope is cylindrical cylindrical in shape. Its Its size is dependent upon the KV that will be applied across the tube. The high KVP tube are larger.


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The ends of the tube are turned upon themselves. The tube is is wider in diameter at its middle position than at its ends. The narrow ends afford support to electrodes and help to permit the sealing of the glass envelope to the metal parts. If the vacuum has been spoilt it may lead to disaster. Whereas the electrons electrons that are to leave the filament filament no longer have unimpeded passage to the anode, but they collide with the atoms of the gas formed formed inside the tube. As result, the electrons loose their energy and result in production of less intense and less penetrating x-rays, when they reach the target of the anode. This cause increase in the tube current erratically and becomes much bigger in proportion that is completely uncontrollable.

3.18 THE TUBE SHIELD:

The metallic case around the x-ray tube provides protection against radiation risk risk and electrical risk. So, it must be ray proof and shock shock proof. The metallic case also, serves to contain and support the glass tube and so so it is often called, the TUBE HOUSING while the glass x-ray tube is called the INSERT in fact the metallic case is not completely ray proof as it is impossible according to the knowledge of physics of x-ray absorption. But it is possible to alter the x-ray beam so that the amount of radiation coming through the absorber is very small and is within the limits of safety (10mr/hr at 1 meter). The tube shield has an aperture covered by a plastic cover through which the useful x-ray beam is allowed to come out to be carefully limited and directed from this portal.


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3.18.1 THE TUBE SHIELD:

Is made by lining the cylindrical Aluminimum or Aluminium alloy case with thin sheets of lead which are capable of absorbing the unwanted x-rays around the tube and effectively effectively reduce the leakage radiation to the the permissible permissible levels. The anode side has less less thickness of lead protection, as its heavy copper mass absorbs most of the radiation.

THE TUBE SHIELD

The tube housing has 2 projections on one of its side which are known as CABLE RECEPTACLES. These 2 projections contain insulated insulated sockets or pots to receive the cables, which connect the x-rays tube to the H.T. Supply. One socket for the cathode cable is on its cathode end side so that the cathode filament can be connected to its cable. The other socket is at the anode end side of the casing so that the anode may be connected to H.T. supply.

3.18.2 THE INSULATING MEDIUM:

The medium chosen as insulating medium is a thin purified oil which is able to serve the double purpose of acting as an electrical insulator and also as a cooling agent. When the x-ray tube is in use, the oil receives the heat produced inside the tube and therefore becomes warmer and expands in volume. Some means must be provided for the expanded oil to occupy extra space without damaging the glass tube. For this purpose a diaphragm or bellow made of synthetic Fundamentals Of X-Ray Equipments

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rubber is provided with in the housing, allows the oil to expand the size of the bellow. When When the oil becomes cooler, the bellow retracts to its original size.

3.19 SHOCK PROOFING:

If the insulation of the equipment breaks down down the high voltage could make the current flow through the body of some one who touches it. The magnitude of shock depends on the strength of the current and resistance of the body of the person the dampness of the skin of the person and whether the person was wearing rubber soled shoes or not. It is therefore very important to make the equipment shockproof by earthing, so that the current in the event of breakdown of insulation, flows directly to the ground and prevents shock to the person.

Therefore all high voltage parts of the equipments x-ray x-ray tube, H.T. Generator the cables which do the job of conducting high voltage current are oil earthed by means of an unbroken conductor of low resistance like copper or G.I. wire. Further shock proofing is achieved by the filament transformer also being earthed for it is connection with the x-ray tube which is connected to high voltage on the the anode side. The controls for the filament current current are included in the primary primary circuit of the filament. The KV control switch also is included in the autotransformer circuit. circuit. Both these are of low voltage in their primary primary windings. So the switches switches in the control panel are shockproof.

In larger and more complex x-ray machines it is not possible to include the x-ray tube and H.T.T. in one tank. So the three three elements x-ray tube is oil oil immersed immersed and its metal metal shield shield is earthed. The H.T. Generator and the stepdown transformer for the the filaments along with rectifiers are together oil immersed in a large large tank. tank. Connections of the tube to the H.T. Generator is is made by special special cables pair which have a metal sheathing which is also earthed.

3.20 HIGH TENSION CABLES:

Its construction is as follows Inner most 3 copper wire conductors are surrounded by insulating material made of special rubber individually and severally held together. This insulation is very thick and hence the cable becomes bulkier and heavier, less less flexible and more more expensive. Around the rubber cable is a flexible metallic sheath made of strands of metal plaited or braided together. This metal sheath is connected to the earth. Finally over the metallic sheath covering is a plastic sheath designed to protect protect the flexible metal braiding from damage.


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HIGH TENSION CABLES

The cables are connected to the tube housing and to to the generator tank by means means of special type of plug and socket arrangement. The cable receptacles in the side side arms of the tube housing hold these insulator sockets to accommodate the cable ends. Similar sockets are provided on the top of the HT tank also which holds the tank end of the cables in position.

COMMON KEY


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These sockets have contacts in their flows which accepts the conducting cores of the cables which emerge as pins from the ends of cables. Each cable has at each end a metallic screw cap which slides loosely over its outer covering and is screwed into place on the tube housing and on the H.T tank after the cable is is properly pushed home into their respective sockets.

3.21 COOLING COOLING MECHANISM OF THE X-RAY TUBE:

All most all the energy put into x-ray tube is converted into heat while only small part (1%) gives rise to exposure radiation. The great amount of heat is related to the product = KV X mA seconds. The greater these factors are, the more heat is developed at the target. If dissipation of heat were not taking place simultaneously simultaneously with heat production, the melting melting point 0 of Tungsten ( 3360 F) will soon be reached.

The process of heat dissipation are in the form of 1) conduction – through the solid solid parts of anode 2) convection – through the oil surrounding the tube 3) radiation occurring through the vacuum of the tube which passes of the heat to glass envelope or from the metallic housing through the air into the atmospheric air in the room. The Tungsten plate passes of most most of the heat developed in it in the the target area by conduction into the copper block of the anode on which the target is set. This copper block has the capacity to receive amount of heat and conduct the heat along its length to the outside, of the tube. The target also conveys a small part of its heat by radiation across, the vacuum of the x-ray tube to the glass envelope.

The glass envelope and the copper block together together transmit the heat to the oil in which the tube is immersed. Oil accepts this large amount of heat (like the copper rod) without itself getting raised in temperature. Convection currents are set up in the oil and the heat is conveyed to the metal casing. The metal casing loses its heat by conversion and radiation into the air surrounding the x-ray tube.

When x-ray tube is used for for fluoroscopy, the tube is in a continuous operation, when the process of heat loss can be hastened by providing an air circulator (fan) which is contained in the housing mounted on the tube shield. shield. It speeds up the heat loss from from the casing by convection process.


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3.22 FILTRATION IN THE X-RAY TUBE.

An x-ray beam is produced with many many wavelengths in it. A filter acts by absorbing preferentially the useless but harmful long wavelengths in the beam. Then the beam emerging out of the filter filter contains shorter wavelength quality. Of these, shorter shorter wavelength radiations radiations produce sharper radiographic image.

Coming out of the tube, after it leaves the target, the useful beam must pass through; 1) the thin window in the glass envelope 2) the oil within the shield 3) the lead lined plastic cover aperture called the portal. All these 3 together add up to a filter (but not in t heir individual capacity) which is is able to remove the longest longest wavelengths in the the beam. This is called the inherent filtration filtration of the tube.

The manufacturer specifies this inherent filtration on the tube shield label in terms of mm of aluminium equipment, which is about 1 mm of aluminium. It is possible to filter the x-ray beam further of the long wavelengths, to render less dose to the patient and give radiographically useful beam. Thus the total tube filtration which includes inherent filtration plus added filtration must be not less than Aluminium of 1.5 mm upto 70 KV than 2.0 mm upto 100 KV than 2.5 mm for above 100 KV x-ray equipment.

3.23 LIMITATIONS OF THE FIXED ANODE TUBE:

A large focal area allows more electrical energy and more heat to be put in without raising the temperature of local spot to the melting point of Tungsten and therefore yields greater radiographic output. On the other hand the large focal area renders the radiographic image image geometrically unsharp which is not noted with small focal area. These conflicting problems are partially resolved by the use of a sloping anode face angle or by the use of dual focus x-ray tubes. There is however, a limit to what can be achieved by the sloping sloping anode face.

A very large focus cannot be foreshortened enough to make it appear acceptably small. An anode angle slight enough to achieve the foreshortening required would result in a narrow useful beam which can cover only small sized x-ray film which would not be practical. This is the limit to to reduction in apparent focal size that can be achieved in practice with the fixed anode x-ray tubes.


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A ROTATING ROTATING ANODE X-RAY X-RAY TUBE

3.24 THE ROTATING ANODE X-RAY TUBE:

The rotating anode x-ray tubes were introduced in the market in 1940’s. The essential difference between the the fixed fixed anode tube (F.A.T.) and this is that the anode is of the shape of disc which rotates. In the Rotating Anode Tube (R.A.T.) the area covered by the electron beam is maintained small but the area over which the heat from the electr5on bombardment being spread is large in contrast to the Fixed anode Tube, where both areas are same. Thus the Rotating Anode Tube permits the use of big electrical loads in combination with effective focus and apparent x-ray source. It is possible to use high mA and short exposure times without sacrificing the radiographic detail.


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3.24.1 THE CATHODE AND FILAMENT

The helical Tungsten filament position is not in the central axis of the x-ray tube, as the cathode structure supporting the filament is in position off centre to the tubes central axis. The electron beam from the filament travels through the tube along a line parallel to the long central axis of the tube and is brought to a focus focus towards edge of the anode disc which faces the filament. filament.

The filament is against housed in the supporting structure so that the electron stream stream is focused, one side of the filament being electrically connected to the cathode structure so that a negative potential is on both. The area covered by the electron stream is determined by cathode characteristic. The size and shape of the filament and the dimensions of the focusing slot and also the spacing of he anode from the cathode.

THE ANODE: The anode of Rotating Anode Tube is a heavy disc mounted on a Molybdenum stem which function as its support Modern discs area made of Molybdenum faced with Tungsten or Rhenium. The diameter of the disc is an important important factor which determine the the permissible electrical load. The ranges of disc diameter are 50 – 100 mm. this disc is not a flat one. Its outer rim is beveled. The discs has its centre at the longitudinal central axis of the tube. During the exposure, the anode rotates about this central axis at a speed of 3000 RPM.


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ROTATING ANODE TUNGSTEN DISC

The electron beam is focused so that is covers a rectangular area towards the periphery of the disc which is beveled. The rectangular area of bombardment is essentially the same same as in F.A.T. but on the beveled rotating edge of the anode ano de disc. This beveling of the disc functions exactly as a sloped angle of he anode of F.A.T and fore shortens this rectangle to a square so that, the 0 0 apparent size of the source is is smaller than actual source. The angle of bevel is 10 to 20 but the 0 modern tubes have a bevel of 10 which gives a true true source source between 5 to 6 times greater than the apparent one. The area over which heat is spread, is a much much bigger bigger one. The rotation rotation of the anode ensures that the position of the disc under the electron bombardment is changing all the time. So the area over which the heat is spread is in the the form of a ring, whose outer circumferences circumferences is equal to the outer circumference. While the ring’s inner –circumference depends upon the electron stream focused. This ring is called the “Focal Spot Track” or “Target Track”.


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REDUCED TARGET ANGLE

This target track is 3 mm wide. As the area over which heat is spread is many times greater than the area covered by the electron beam, the tube will allow much more heat to be put into it and will take bigger electrical loads. The greatest advantage R.A.T. is that it allows high mA to be used to get the best radiographic detail.

3.24.2 ANODE SUPPORT AND ROTATION:

The anode disc is supported by a stem made of Molybdenum. One end of this stem is attached to the disc at its centre and the other end is mounted into into a copper cylinder. This copper cylinder is known as ROTAR because it is the rotating part of the electric motor which provides the rotating force. The rotor rotates because electrical currents are induced in it and it rotates because electrical currents are induced in it and it rotates on its own support which emerges through the end of the glass envelope. The emergent of the rotor support is used to connect the high KV supply to the anode of x-ray tube. The glass envelope is sealed to the rotor support with a vacuum tight, heat proof Fundamentals Of X-Ray Equipments

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seal. The rotation takes place on ball bearing between the rotor and its support. The ball bearings are made of steel and are specially lubricated. It is seen that the heat does not reach the ball bearing to ensure long life for it which is very essential for life of R.A.T. This lubrication of bearings is obtained by silver / lead coating. The force of rotation for the anode is obtained from the electric motor of induction type. The rotor collar outside the glass envelope carries the iron core of the motor and its electrical windings. This core and winding together form the the STATOR as this is the static part of the electric motor. When the motor is is energized, the windings carry alternating current current and the rotor is with in the magnetic field from this current. Since the field field from the windings rotates, the rotor fields wants to follow it round and the rotor rotor freely rotates.

When the R.A.T. is in use for radiography, the anode must be rotating during exposure at its full speed of 2400 or 3600 RPM. Before the exposure begins. It will take about 1 second for the anode to reach this this speed. The manufacturer gives the design of the exposure switch witch such that the first mild pressure on the switch makes the anode to rotate described as prepare period and then the second mild pressure starts the exposure only after the anode is rotating at its full speed.

It the radiographer tries to evade this prepare period by depressing the exposure button deeply at once without doing it in 2 stages, it will be found that the exposure in any case will not being, until the end of the delay imposed to allow the anode to reach its full speed of rotation. When the exposure stops. If If the rotation of anode or the prepare stage does not occur when we press the Fundamentals Of X-Ray Equipments

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exposure button then there is some thing wrong and needs attention and so no exposure must be attempted. When special work is undertaken like rapid sequence sequence serial radiography radiography and cine angiography, the anode can be put into continuous rotation independently of exposure switch by a special circuit and de-linking de-linking arrangement on the control-panel. But one should should always remember remember to switch off the anode as soon as possible in these special works. DUAL FOCUS ROTATING ANODE TUBE (D.F.R.A.T.)

Modern rotating anode tube are dual focus type. type. In these, the cathode structure supports 2 filaments side by side or one above the other, one of them being larger than the other, the area covered by the the electron beam from each filament is is determined by the shape and size of the filaments and the dimensions of their focusing slot. The 2 filaments cover areas areas on the anode which may be superimposed, may may be placed side by side or may be placed so that one is nearer nearer the centre of the anode disc. 3.24.3 THE GLASS ENVELOPE

Glass envelope of the R.A.T is not of the shape of F.A.T the envelop of R.A.T is narrow at its extremities where it is turned on itself to form, the the vacuum tight seal with the metal parts. At the anode end, the glass envelope forms a long narrow neck to accommodate on its outside the stator core and its windings, and on its inside the rotor rotating freely very close to the the glass. Just about where the inner end of the the rotor narrows to the the molybderum molybderum stem, which supports the anode disc, the glass envelope widens to becomes a larger cylinder. The central part of the envelope is of much bigger diameter than the extremities. Beneath the anode where the x-ray beam emerges, there is a thinner window. The other places the glass is thick, hard and heat resistant glass is used for the glass envelope of the tube. The vacuum inside the tube is near perfect. Pure material are selected and the degassing procedures are carried out with the tube and its parts raised to the high temperatures. temperatures. 3.24.4 THE TUBE SHIELD

Shield of R.A.T. are similar to those of F.A.T but differ in that they carry a mains voltage supply in addition to energizing the stator stator windings apart from from the 2 cables mentioned earlier. earlier. This cable for stator enters the shield at the anode side end plate of the casing. COOLING OF THE R.A.T. it must be noted that the heat from the tube must be prevented from reaching the ball bearings because, if these ball bearings were to get heated will obstruct smooth rotation of anode. This is prevented in the following manner: 1) Anode disc is mounted on a narrow stem made of a metal which is not a good conductor of heat.


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2) A dummy disc is mounted at the the back of anode disc which serves as baffle reflecting the heat, preventing it from a reaching the rotor and its ball bearings.

3) Heat which reaches the rotor still is taken away by the glass over the blackened outer surface of the rotor by radiation. Due to the the poor conduction of heat from the anode disc, it becomes hot at once, when heavy loads are applied. The anode is able to lose heat by radiation to the glass envelope env elope once the heat reaches the glass envelope env elope by radiation. The heat is lost to the cooling sequence of oil surrounding the glass envelope by conduction and then to the metallic case by convection and by radiation to the atmospheric air of the room. 3.24.5 FILTRATION IN R.A.T.

Inherent and added filtration in x-ray tubes of R.A.T. filtration ranges from 1.5 to 2.5 mm of Aluminium.

are the same as F.A.T

The total

3.24.6 RATING OF X-RAY TUBES.


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The manufacturer provides certain statements of the conditions in which the x-ray tube may safely be used. The details are of electrical loads, temperatures to which focal area can be raised, the cooling rate of the anode, the thermal capacity of the shield, heat dissipating capacity of the shield. The rating of the x-ray x-ray tubes are given under following headings

1) RADIOGRAPHIC RATING: Statements regarding electrical loads which may be safely applied for radiographic exposures and exposure times – these data are given an graphical form.

2) THERMAL RATING: Statements on how much much heat can be safely put into the tube as a whole, to anode and at the rate at which the tube and anode loses the heat. There is also statement regarding the number of total exposure that can be done continuously as in angiography.

3) FLUOROSCOPIC RATING: Statements on the tube may be safely use for fluoroscopy with the tube running continuously, being energized for periods in minutes.

3.25FAULTS IN X-RAY TUBES:

The faults faults in a x-ray tube may may be in 1) the glass envelope 2) the anode, its rotor, its stator windings 3) the cathode filament 4) its vacuum. 3.25.1 FAULTS IN THE GLASS ENVELOPE: ENVELOPE:

On long use the colour of glass envelope may change due to staining forcing a mirror mirror like surface on the inside this is due to repeated heavy exposures causing vaporization of the metal parts. The vapours of Tungsten are deposited on the glass. This deposition of metal film on the inside of glass envelope prevents the tube from conducting its heat to its outside which causes the puncturing of the tube leading to spoiling of the vacuum. Even rough handling can cause fractures in the glass envelope. Some times times stress stress fractures can develop spontaneously. So care must be taken while moving moving the tube column and the resting resting of tube while not in use etc.


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3.25.2 FAULTS IN ANODE, ROTOR AND STATOR

a) ANODE: Every exposure heats target area to a high temperature. Repeated exposures leave the originally smooth surface of the target track eroded and cause crazy paving of the target track. This occurs due to the tube being repeatedly repeatedly loaded above the safe radiographic image. Over heating may also result in permanent distortion of the anode disc which will wobble, when the rotation rotation of anode starts. starts. This means wavering x-ray source and unsatisfactory unsatisfactory radiographic images. b) ROTOR AND ITS BEARINGS: Once the bearings have begun to deteriorate the situation may become steadily worse since its state is vital to smooth rotation of anode at its proper speed (the bearings are affected by heat reaching it from the anode).

c) STATOR AND ITS WINDINGS: If the fault develops in the stator windings there is no power to rotate the anode. A careful radiographer can easily notice the absence of preparatory period of tube rotation soured. Manufacturers however provide a circuit to protect the tube from exposing without rotation of the anode, in the absence of which there is likely.-hood of overheating of the anode and can result in in cracking of anode disc right through.

3.25.3 FAULTS IN CATHODE FILAMENT:

Failure of the filament filament to heat up when when its circuit is energized may be due to : 1) A break in the filament itself itself 2) Due to the fault in the filament circuit The filament, as it ages due to constant use, it becomes thinner and eventually breaks. Then the x-ray tube fails to work. work. The wearing down of the filament, takes place rapidly if the the filament is loaded loaded always by high mA. Filament Filament life can be extended if its energizing for shorter periods (prepare button button position for periods not more than 1 Sec.) and use of the filament for the required mA Value (in filament boost) if the the filament filament is not heating there will be no indication on the mA meter and no exposure of the x-ray film. An intermittent failure of mA meter (coming and going) means filament fault, filament circuit fault or the vacuum is not proper.

3.25.4 FAULTS IN VACUUM

When vacuum is spoilt, the mA meter shows erratic values of high levels than that is set. If still the exposures are made, arcing will take place and resulting resulting explosion. The cause for this this loss of vacuum may be the use of x-ray tube which has become gassy resulting in minute fracture of glass envelope. Fundamentals Of X-Ray Equipments

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3.26 NEW DEVELOPMENTS IN X-RAY TUBES:

The following different types of changes have taken place ever since R.A.T has come into vogue. 1) 2) 3) 4)

0

Target angle less less than than 15 Anode disc composition (bimetallic combination) Faster speeds of anode rotation A form of switching for H.T. supply to begin and end the exposure which is in the x-ray tube itself.

3.26.1 REDUCED TARGET ANGLE:

The smaller the target angle the greater is the electrical load which can be put on the x-ray 0 0 tube. It has been found that smaller target angles of 10 and 7 are possible. An anode of double angle disc has two focal target tracks arranged as two concentric circles on the disc, the outer track for broad focus, the inner track for for fine focus. focus. The surface of the disc is beveled at two angles for the above purpose. 3.26.2 NEW TARGET MATERIALS:

The new material for target of anode is is an alloy of Rhenium and Tungsten, which is used to face the anode disc. This new material resists the roughening process due to repeated use, use, better than the pure Tungsten target and so prevents faster deterioration. Another material now in use is the combination of Tungsten and Molybdenum, as it has got the ability to accept huge heats without itself getting raised in temperature. And therefore bigger electrical loads are possible than before. This is of importance because the conduction to ball bearings is reduced when when molybdenum - Tungsten discs are used. 3.26.3 SPEED OF ANODE ROTATION:

Now a days with disc diameter of 4” (10 Cm) rotating at 12000 RPM the tube can take maximum loads. For the achievement of these speeds, the AC supply of 50/Sec is altered into 150/Sec. An Electronic device does this conversion of 50 to 150 per second to the stator supply. 3.26.4 GRID CONTROLLED X-RAY TUBES:

This is a new method of switching on the the x-ray tube by itself, itself, which allows extremely extremely short exposures to be used since the positive voltage can be applied as a pulse for a brief duration. These short exposures can be repeated fastly, one after another, without interia of preparation. preparation. Fundamentals Of X-Ray Equipments

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This works by changes in the electrical characteristics of the newly incorporated wire mesh grid across the opening the cathode grid is originally kept negatively biased and when this negative bias is reduced by applying positive voltage to he gird, the cathode is allowed to discharge electrons. Some of the modem x-ray tubes have gird control for fi0ne focus only.

3.26.5 CHOICE OF X- RAY TUBES:

New ranges of x-ray tubes makes one do the selection of x-ray tubes by choice and so care must be applied to the following combination depending upon the requirements for different purposes. 1) 2) 3) 4) 5)

For For For For For

0

Neurography H.U.C. – 300,000, T.A. 7 ; F.S.-0.3 AND 0.6 mm 0 Angiography H.U.C. – 400,000, T.A. 12 ; F.S.-0.6 AND 1.2 mm 0 General Radiology H.U.C. – 300,000, T.A. 12 ; F.S.-0.3 and 1.2 mm 0 General Radiology H.U.C. – 300,000, T.A. 15 ; F.S.-0.6 and 2.0 mm 0 Mammography H.U.C. – 300,000, T.A. 10 ; F.S.-0.6 mm

3.27 FURTHER DETAILS OF X-RAY BEAM:

X-rays coming out of the tube tube window are known as X-ray beam. This x-ray beam is divided into two parts (a) The primary beam which is heterogeneous beam of x-rays emitted from focal spot of the tube and enters the body. It is altered in its intensity and also in its quality as it traverses the body tissues. As it emerges out of the beam, the beam is called “The Remnant beam” of x-rays which is exposed on to the x-ray film and forms forms the image of the part radiographed. This remnant beam will also contain certain amount of secondary radiation of x-rays emitted after absorption by the body tissues. b) Central ray: This is the centre of the primary beam of x-rays. This is of importance in radiography so as to denote the line of projection of x-ray beam. The course of this central ray is from the focal spot to the centre of the x-ray film. So in centring before exposure of a film film three things are to be considered. 1) The line or axis of the central ray. 2) The centring on the part of the body under examination. 3) The centre of the film. All these three three must be in line of projection to get the best radiographic results. There are devices in the tube head to indicate the line of central ray (i.e. collimator and telescopic type of centring device.) 3.28 ANODE HEEL EFFECT:

The x-rays are produced in all directions directions from the anode target. But they all are not of equal intensity in all directions. Again, it is is noticed that the x-rays produced in the directions directions nearer Fundamentals Of X-Ray Equipments

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the anode are less in intensity intensity and quantity than those produced in the direction away from the anode. (i.e. those in the cathode side direction. This phenomenon is known as anode heel effect. This principle is used to get better radiographs of parts of the body of uneven thickness or of uneven densities (i.e. L.S. Spine-lateral view). Here the cathode side rays which are of more intensity and quality are directed towards the thick part to get best radiographs. 3.29FILTERED RADIATION:

The x-ray tubes have a tube shield which is so well made that it will not allow any x-rays to escape out of the the x-ray tube except through the glass window which is is provided to allow only the primary beam from the focal spot. Modern x-ray tubes and shields are so constructed that they permit only 8 milli-rontgens per hour, from the back of the tube shield, the glass window itself filters certain long wave length x-rays of the primary beam and this is called the “Inherent Filtration” whose filtration effect is equivalent to 2mm of aluminium, as already discussed.

In addition, the tube shield has slot to carry further further Aluminium Aluminium filter. By increasing increasing these filters in thickness the x-rays of harder nature only (i.e. short-wave – lengths) are permitted to pass through these filters which are of high diagnostic value. These filtration tend to reduce the skin damage damage that can be caused by x-rays x-rays of longer wavelengths. So the the filters filters are protective devices and radiography or fluoroscopy must must not be done without any filtration at all. all. 3.30 SCATTERED RADIATION:

When the x-rays strike the body part, secondary radiations are produced which have longer wave lengths than the primary beam. These secondary radiations are scattered in all directions and produce a veil of fog on the diagnostic x-ray film emulsion. This fog spoils the contrast of the radiogrpah, rendering it it poor in definition of image. These unwanted scattered rays rays can be cut off by various devices during radiography, to get better image with proper contrast and sharpness of details. Secondary radiations do not affect the image of the part of the body nearest the film but affect the image of the part distance from the film by the fogging. Scattering again depends on the thickness of the tissues tissues through which which the x-rays x-rays have to pass through. The greater the thickness, the more is the secondary radiation. Small parts like hand and feet show negligible scattering but pelvis and trunk show maximum scattering.


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3.31 CONTROL OF SECONDARY RADIATION: Some Radiographic accessories are used to cut down the secondary secondary radiation fog. One of them is to reduce the amount amount of field of tissues exposed to the correct required level, by the use of CONES, SHUTTER DIAPHRAGMS or GRIDS. These gadgets help to limit the field of exposure by improving the contrast and image image details. The shutter diaphragms have lighting lighting arrangements above them, them, incorporated inside the tube shield which show the field size (after operating the shutters) on the patients body.

Thus both in in radiography radiography and Fluroscopy, it is possible to reduce reduce the harmful secondary radiation to the patient and doctor, apart from getting image of good contrast, cones of different sizes and shapes are available to restrict the field of radiography to to the abso9lute required size, as in the case of mastoid cones and PNS cones, which are named after their main use. These cones can also be used for other regions for for localized cones own views of radiographs, such as for pituitary fossa, individual vertebra, for better details.

DENTAL CONES


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BEAM REETRICTING DEVICES

3.32

GRIDS

These are devices which consist of series of alternate alternate strips of lead lead (or Tungsten) and wood (or similar radiolucent material) These strips are so constructed constructed that they are deeper than wider and are arranged in a segment of a circle with with a focal spot spot of 36 to 48 inches. In other words, the plane of each strip is in the plane of the primary beam.


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The x-rays penetrate the wooden strips and fall on the film to produce the image. The x-rays of longer wave lengths (the scattered x-rays) are cut off by the lead strips. So, when a grid is interposed between the patient and the film, this allows allows only the remnant beam rays in the line of the radius of the segment of the circle, whereas the scattered scattered x-rays in the other directions are cut off by the lead strips. Grids are are of mainly 2 types – 1) stationary stationary and 2) moving grids. The stationary grids are also known as LYSHOLM GRID. The moving moving grids are of three types 1) potter – bucky grid 2) oscilating grid 3 ) cross grid.


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3.32.1 STATIONARY GRID:

In this, the grid lines are visualized on the radiograph but do not actually interfere with the image. It appears as number of fine lines evenly spaced on films, containing about 60 to 80 lines per inch. Some Some latest latest ones have 110 grid grid lines per inch, and are made of Tungsten strips which are not visible at all unless closely scrutinized. These stationary grids are used only when the patients cannot be x-rayed in Radiography Department on the larger x-ray machines which have in built grids but have to be x-rayed only in the wards for abdomen and pelvis or skull x-rays. 3.32.2 MOVING GRIDS:

1) Potter Bucky Diaphragm: The problem of gridlines on the radiographs have been removed by moving the grid while the exposure is going g oing on. The movement move ment of the grid during du ring exposure is brought about by hydraulic pressure or suitable mechanical methods with the help of a motor specially meant for this purpose. The use of Bucky grid necessitates the increase of the exposure time 3 to 4 times (mAs) This is known as Bucky factor. The strip of lead and wood each are about 0.005 inch in thickness. Grid ratio denotes the relation between the height of the lamella and the mutual distance between each lamelle. lamelle. Normally this is about 8:1 the higher higher


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the grid ratio, the greater is the efficiency efficiency is absorbing scattered radiation. So a grid of is used for high KV technique radiography.

2) Oscillating grids or reciprocating bucky is similar similar to potter bucky diaphragm but this swing swing like a pendulum to and fro fro across the the table below it, continuously through out the exposure. The th time taken for curing back is 1/100 of second. This gives better contrast.

3) Cross Grids: In this, there are 2 grids grids mounted over each other. During exposure these these 2 grids move diagonal to each other and remain even on one another when they are idle.

3.32.3 CHOICE OF GRID:

In stationary stationary grids, the high value number of grid grid lines must be chosen. But on the other hand in moving grids the ones with low value in number of lines must be chosen. Focus-grid distance distance plays important role in image contrast. Further as a result of incorrect centring, there occurs loss of primary radiation which can result in loss of contrast. For a picture of 30 cm size a horizontal decentering of 1 cm will result in 50% loss in contrast at the edge of the film in the linear grids


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and about 25% loss in the cross grids. Selection of grids depends on focal distance, number of grid lines and grid ratio. ratio. Bucky factor, the total lead content in the grid small bucky factor (small exposure time) The Bucky factor is the ratio of exposure time WITH BUCKY to that WITHOUT BUCKY. The exposure time time can be cutdown by increasing increasing the the voltage, voltage, which makes the bucky factor small.

3.33TUBE STANDS, CEILING CEILING TUBE SUPPORTS:

The function of a tube stand is to to support the x-ray tube so that it can be applied by the Radiographer for for Radiography. It is important item in the x-ray equipment and has to fulfil certain requirements for the x-ray tube to be used easily and freely. freely. 1) The supports must be adequately rigid to avoid any vibration vibration of x-ray tube 2) Any movements of the support and of the x-ray tube about it must be smooth, unrestricted and easy to perform. 3) It must be possible to make certain precise angulations of x-ray beam. 4) It must be possible to direct the the x-ray beam parallel to the floor as well as in in perpendicular direction. 5) The controls for providing the tube movements should be readily accessible Tube supports are mainly of two categories 1) vertical tube stands or column 2) Ceiling suspension system.


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3.34 TUBE STANDS:

It consists consists of a vertical column of heavy gauge steel tubing which is mounted on a carriage to move on tracks on the floor and ceiling. The floor track is often recessed. On the vertical column, a cross arm supports the x-ray tube.


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FAIL - SAFE DEVICE FOR THE SUSPENSION CABLE

This cross arm is on ball bearings and can be moved a) up and down the the column b) at right right angles to the column c) and in a rotational motion about the the vertical axis of the column. Again the x-ray tube itself can be rotated rotated upon the cross arm and also tilted tilted about an axis parallel to itself.

By the

above mentioned mentioned provisions, the following excursions of the the x-ray x-ray tube is

possible.

1) Longitudinal travel is possible usually upto 3.5 meters or 12 feet or even further further to the limits of the length of the room.


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2) The horizontal travel is restricted to 90 cm (3 feet) due to length of the cross arm itself and in the other directions by the the configuration of the bracket holding the x-ray tube. It is not possible to lower the x-ray tube lower the x-ray tube tube closer to the vertical stand on the cross arm than about 30” or 75 cm. However this movement on the cross arm is enough for Bucky tube work / stretcher work/ chest stand / vertical bucky work. work. The

3) cross arm carries a scale which is is calibrated in centimeters or inches. Often there is a click when the tube is in the centre on the cross arm.

4) Vertical travel up and down the column depends on the height of the vertical column. This usually affords a maximum F.F.D. of 1.25 m (50”) and in some cases 2 m(6’). It is possible possible to lower the tube only to 30” (75 cm) from the floor. There is scale fixed to the vertical column to indicate this distance.

5) Rotational travel about the vertical column. Some equipments permit the rotation of 0

0

360 . Some manufacturers allow only 270

0

to 300 rotation, in the interest of H.T. 0

cables. Often automatic locations of the tube are provided at 90 intervals and a manual control is used to lock the tube column at any intermediate angle. 0

0

6) Rotation on the axis parallel to the cross arm from 180 to 180 . Freedom of rotation and precise movements on this axis are important to the radiographer since many techniques require accurate angulations towards feet or head of a patient who may be lying or sitting at a table bucky / vertical bucky. Angulations scales are provided by 0

manufacturers, marked at intervals of 1 and

is prominently situated on the tube

movement.


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7) Rotation round the tube’s own long axis: this is necessarily restricted by the the nature nature of the bracket holding the tube tube and will vary in extent extent with the details details of the movements movements design. 0

In one example of tube can be rotated up to 30 0

on either side of the vertical. The other

0

example is 125 to one side of vertical vertical and only 20 to the the other side of vertical.

3.35 THE BRAKES:

Electromagnetic brakes operated by punch buttons or switches switches on the tube mount are usually provided for the 3 directions of travel longitudinal on the floor track transverse on the cross arm and vertical column. There are also additional manual locks to be used when the mains supply is off. In some some instances, circuit is incomplete and are released released when the circuit is made. This means The means that when the mains supply is off the tube will will not be pulled down by the counter weight of H.T. cables and results in its damage.


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3.36 TUBE SUSPENSION AND COUNTER WEIGHTING:

The cross arm carrying the tube is suspended by a variety of means 1) a single single wire cable employed 2) Twin wire cables employed 3) A dual system of a cable and chain. The tube counter weighted for its vertical motion, the counter weights being usually placed within the tube column. In some cases the tube is counter poised by means of springs within the column.

TUBE SUSPENSION AND COUNTER WEIGHTING


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Where the suspension is by means of a single cable a fool proof mechanism is needed to prevent a rapid descent de scent of the cross arm, when the cable snaps, to avoid injury to tube or o r to the patient on the table. In the other type, the tube column is grooved on one aspect to provide a channel in which a brake block with in in the carriage casting of the cross arm can move. A spring spring fixed at one lower corner of the brake block provides a force which would push it out of alignment in in its groove. Thus, arresting the downward rush of the carriage cross arm of x-ray tube.

3.37 CEILING TUBE SUPPORT (TUBE HANGERS)

Arrangements which supports the x-ray tube by a suspension from the ceiling are available in a variety of designs. These can be ceiling ceiling suspension or ceiling cranes. These mainly mainly consists of: 1) A main main supporting unit or carriage from from which the x-ray tube is is suspended by a telescopic system. 2) A pair of rods on which the tube carriage travel in the direction designated for transverse transverse (at right angle to the table). These rails are actually a second second carriage system or bridge. 3) The longitudinal tracks which are fixed usually to the ceiling of the room or to the side wall or both. 3.38 TUBE MOVEMENTS – THEIR CONTROL 3.38.1 HORIZONTAL TRAVEL:

Area covered by longitudinal and transverse transverse travel may be equal with in area from controlling electromagnets operated by push button, button, fix fix the tubes. Pilot lamp is provided to prevent the movements of the tube when the table is is under motorized movements. This prevents any injury to the tube by the movements of table. 3.38.2 VERTICAL TRAVEL

(9 TO 10 Ft) the joints which carry the ceiling tracks are fixed to the ceiling. Maximum film film tube target distance is is 1.5 meter or 5 ft. minimum 50 cm (20”) 3.38.3 ROTATION

a) about the axis of the telescopic hanger b) about the the transverse transverse axis of the tube.


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3.38.4 ADVANTAGES:

One x-ray tube can serve 2 or 3 x-ray tables or a patient who is brought in the bed or stretcher. Teleradiography is easy 1) Alignment with vertical bucky is quick. Exposure using a horizontal beam (lateral view for femoral neck) can be done with ease and thus creating much moving space for use in the room. Access to table is from from all sides. Tomographic attachment is also easy and is without any vibration. 3.39 ULTIPURPOSE ULTIPURPOSE TABLE FOR RADIOGRAPHY AND FLUROSCOPY:

In x-ray department the x-ray table must be of multipurpose and be able to be employed for both fluoroscopy as well as most of the general radiographic examinations at different times and also for fluororadiography at the same time. Besides the table must be useful for tomography, skull radiography, angiography etc. the multipurpose tilting table is also referred to as universal table.

3.40 GENERAL FEATURES OF THE TABLE : listed below:

Important and common features are

1) A drive of some kind hand driven or motor driven, so that the table can be used in horizontal or vertical or in other angular position. 2) An x-ray tube beneath the table (under couch tube fitted) associated with fluoroscopic screen attachment above it which are able to be moved together. 3) An apparatus called the spot film device which is attached to the fluoroscopic system that permits the radiography during fluoroscopic examination. 4) A bucky tray mechanism in the normal place beneath the table top and above the cassette tray. 5) Occasionally for reasons of economy one single tube is used to perform both fluoroscopy and radiography. The table base is a heavy structure. It must must support the weight of the body of patient as well as the body of the table, the under couch tube, the fluoroscopic screen and the spot film, device, together with the counter weight system belonging to these attachments, besides bucky and its counter weights. The base also provides PIVOT POINTS on which the body of the table can tilt tilt and it often houses the motor device. The dimensions of these tables vary. But a typical model may be of following dimensions. 1)width 75 cm(30”) 2) length – 200 225 cm (80” 90”) 3) height – 85 cm (32” - 33”) The sides of the table are enclosed with panels of sheet steel painted smoothly. The material used for table top is necessarily radio transparent i.e. Lamented plywood, plywood, Bakelite or prespex. A foot rest on which the patient can stand is is also provided with the tilting table, when not needed.


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3.41. TABLE DRIVE AND TABLE MOVEMENTS:

The table tilt drive may be from an electric motor or an electro hydraulic mechanism. A 0 movement of the table through 90 in 20 – 30 seconds is customary. The motor may be of 2 Speeds, to tilt even quickly, when necessary (e.g. Myelography). The range of tilt available 0 in a tilting table is described as being from the vertical to 55 head down below horizontal (the 0 trendelenburg tilt) The footward tilt is possible for 90 from horizontal. It is obviously necessary to support the patient during these tilting manoeuvres and some supporting accessories are needed on the table, which include shoulder rests and hand grips. The table will will stop automatically in the maximum trendelenburg position and in vertical position. In some cases, it will automatically stop or pause at the horizontal as well. These movements are controlled controlled by switches on the side panel of the x-ray table.

TABLE MOVEMENTS


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If mishandled, tilting tables can cause dangerous results. Besides being risk to to the patient in the dark during fluoroscopy, fluoroscopy, these dangers may be in the form of, the cables being pulled off from their positions, the cast – iron foot rest breaking. There are some equipments which have automatic brake switch mechanisms to stop whenever any obstructions is encountered. There are other models which will tilt only as long as the pressure is applied on the concerned button switches.

Most apparatus provide motor driven movement for the sliding table top on the base longitudinally, and in a few cases transversely transversely (widthwise) as well. These are to help in easy positions of the patient vertically, horizontally and transversely. This sliding table top also makes the the table useful for for rapid serial film / cassette cassette change equipment. The extent of table top travel varies between 30” (75cm) to maximum of 40” (100 (100 cm) cm) at the head end or foot end.

3.42

THE FLUOROSCOPIC COMBINATION.

SCREEN

HOLDER

AND

THE

X-RAY

TUBE

The function of the fluoroscopic screen holder is to contain the fluorescent screen, its associated controls and the accessory apparatus and controls for the radiography during fluoroscopy. The screen has a square format, usually, 14” x 14” (35cm x 35 cm) and is maintained by the screen holder in a plane at right angles to the x-ray beam. In the modern apparatus the under cough x-ray x-ray tube is coupled to the screen holder at one side of the table, so that it cannot be moved independently of the screen. In all positions of the x-ray tube, it remains centred to the centre of the screen. Further the lead glass in front front of fluorescent screen gives the observer protection from radiation. radiation. The coupling can be moved back and forth, up and down and also from side to side. The under cough tube is is fitted with adjustable diaphragm similar to those associated with the light beam delineator of the over couch tube (with no lamp). These diaphragms can be adjusted mechanically or electrically by switches on the the screen holder. At any selected point the screen can be locked in position position by electromagnetic brakes, operated by switches on the screen holder. After compression during transverse movement and longitudinal movement. In addition to the controls described, the screen holder includes mechanism of switches switches of serial spot film device. For the purpose of protecting the observer from scattered radiation from the patient’s body, a lead rubber screen is attached holder to hang below when the table is vertical. This lead screen can be slided to the slide when the table is horizontal.


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In majority of general hospital x-ray departments, the x-ray units are mainly used for radiographic purposes most of the time and only sparingly used for fluoroscopy these days. During general radiography the fluoroscopic screen holder is kept in the retracted or parked position at one end of the table tab le not interfering with movement of vertical tube column, so that it is not damaged and at the same time is readily accessible for use by radiographer or radiologist when needed. Unless the vertical tube, column is brought to one end of the track, the under couch tube and screen holder cannot be put into use. This is a fool proof arrangement to avoid damage to the apparatus, while tilting the table during fluoroscopy in the darkness. This makes sure that the screen holder does not dash against tube column or over couch tube. 3.43 THE SERIAL FILM SPOT DEVICE:

The purpose of serial spot film device is to allow the filming of the fluoroscopic study as and when required by the radiologist quickly. This is an important important aspect of G.I.T. radiology. The serial spot film device (S.S.F.D.) is essentially a lead lined RECESS CONSTRUCTED as an integral part of the screen holder. It is situated to the right of the fluoroscopic screen with which it forms a continuous lead protected tunnel except for the area area behind film screen. THE FUNCTIONS OF SERIAL SPOT FILM DEVICE: When a radiograph is to be taken during fluoroscopy, it is needed to move the cassette quickly to the exposure field. In addition. There are a number of other aspects associated with the x-ray tube and generator that must automatically occur before the film may be exposed. These are as follows 1) Fluoroscopy Fluoroscopy must be switched off 2) the rotating anode of the under couch tube must be set in motion and be brought to full speed for radiography 3) The tube filament current must be boosted to the higher radiographic values 4) Fluoroscopy is done with very small focus (of 0.3 cms) But for radiography we need large focus as we use heavy tube tube current and short intervals intervals of exposure 5) when the exposure (time x mA x KV) occurs it must be in accordance with factors present on the control panel by the radiographer.


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These adjustments and changes occur automatically on the pressing of the exposure switch on the serial spot film holder, after the factors have been already preset on the control, panel by another switch named named “serial remote” in order to make exposure feasible from the operation of a single switch on the spot film holder. This circuit bypasses the usual exposure switch of the control panel. These events take place after a certain certain interval of time, time, so as to enable the acceleration of tube anode, which normally occurs in 0.8 sec. 3.44

CUMULATIVE FLUOROSCOPIC TIMER:

This is a device attached to the equipment which indicates the total period of irradiation of the patient. It operates only when the fluoroscopic screening is going on. It records the period in order to limit the total period of fluoroscopy of any particular patient not to exceed limit of 5 minutes, beyond which the radiation is hazardous to the patient. Before fluoroscopy is started the rotatory disc disc of this timer device is turned turned normally until the point is at ‘0’. Now, when the fluoroscopy is on, the pointer moves to 1 Mt. 2 Mt and fraction there on. It stops as soon as fluoroscopy circuit, at the end of total 5 Mts fixed time, the timer breaks the fluoroscopic circuit and prevents further fluoroscopy until and unless the timer is reset. Some times a buzzer circuit is opened at one end of the prescribed time to warm the operator. In some equipment, the warning system system may be by switching on of a signal lamp.

3.45

REMOTE CONTROL

Of the fluoroscopy table is possible possible and this can be achieved only where the fluoroscopy is done by image intensifier connected to a closed circuit TV monitor, when the doctor sits in nearby room capable of watching the patient through a lead glass window. He operates the various switches controlling controlling the movements of the table top. Patient rotating equipment and other control switches are available by his h is side in the console. The fluoroscopist uses his rod vision or peripheral vision to study the image. So the fluoroscopic room must be a thoroughly dark one. Further it is important that that the fluoroscopist adapts his eyes for darkness for at least 20 – 30 Mts before he starts the fluoroscopy. His visual visual acuity and integration time play important role. If an image is not studied and understood with in a few seconds of screening, probably it is is not going to be studies at all. By looking slightly to the side of point of interest, interest, eyes are to be kept in constant motion along the screen movement as these improve rod vision.


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3.46

CONSTRUCTION OF FLUOROSCOPIC SCREEN:

Tungstate was initially used as phosphor between 1914 and 1933, Patterson “B” type screen have come in to use, since then. In this, phosphor is made of larger crystals of Zinc Cadmium sulphide, which is silver activated. This screen is more efficient. The phosphor layer is thicker than that of the the intensifying intensifying screens. It is 7 to to 8 mils in thickness. thickness. The phosphor particle size varies from 25 to 40 microns. The resolving power of fluorescent fluorescent screen is inferior to that of Intensifying screens (3 time pairs /mm) /mm) There is a protection lead glass on the observer side side of the phosphor layer which is transparent enough to see through and also protective enough to absorb the x-rays x-rays of primary beam efficiently. 3.47

INDICATIONS FOR FLUOROSCOPIC EXAMINATION

Include: 1) 2) 3) 4)

To study the heart movement in pericardial effusion effusion or constrictive pericarditis. To study the seesaw pulsations of aortic knob in Aortic regurgitation regurgitation To see the the hilar-dance in cases of A.S.D. To study study the oesophagus in in oesophageal pathologies and also in the assessment of cardiac chambers enlargement. 5) To study the motility of stomach in Barium meal examination and the study study of duodenum. 6) To rule out diaphragmatic hernia or hiatus hernia 7) To study the bronchial tree while doing bronchography 8) To study the pharynx for any pharyngeal pathology 9) To assess colonic filling during the Barium enema examination 10) To study the spinal and cerebral CSF pathway during Myelography and contrast ventriculography.

3.48

THE BUCKY ASSEMBLY:

This is an integral part of the x-ray table. This incorporates 1) a frame which holds the grid and also allows interchange interchange of grids as and when necessary 2) the grid itself (46 cm x 43 cm) 18” x 17” in size 3) the grid movement mechanism 4) a robust steel tray underneath the above mentioned items, items, which can hold any size cassette. This tray also must be easily movable.


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In a typical bucky table, the whole arrangement is amounted on bearings which allow allow it to move along a pair of rails rails along most of the the length of table top. top. The grid and film film can thus be positioned together in any appropriate place beneath a recumbent patient on the table. A grid operated lock is also provided to hold the bucky at the selected site. The grid and its movement are not visible to the operator unless the table top is removed. The bucky assembly is mounted on a pair of steel column and is counter weighted and counterpoised on its support. We mainly choose either a reciprocating movement or oscillating movement.

THE RECIPROCATING MOVEMENT OF GRID IN THE BUCKY: BUCKY: When When operated by a reciprocating movement, the grid is driven to and from one side of the cable to the other, during x-ray exposure. The grid is propelled in one direction by the combination of 2 springs springs and a speed kept constant by controlling with an oil dash-pot (Consisting of a barrel and plunger device with oil) The traverse of grid across the film is no more than a few centimeters in extent. If the traverse speed is inadequate gridlines are noticed on the radiograph. A very high speed traverse will cause vibration of the bucky assembly causing unshapness of radiographs. Optimum speed control with smooth smooth movement is accomplished by the oil dashpot and its adjustment knob. The grid in operated by solenoid in the bucky circuit.


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THE OSCILLATING MOVEMENT: When the oscillating movement is used the grid moves to and fro across the film throughout the exposure. The S movement is not so much an implied one but is obtained by merely mounting mounting the grid on a spring spring at each corner and giving it a push from time to time time causing it to vbibrate upon springs. springs. These springs springs are leaf spring spring type made of strong flexible steel springs and is mounted in relation to the four corners of grid in frame. A movable bar or lever lever is operated by a solenoid through a bucky circuit. AN OSCILLATING BUCKY MECHANISM


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3.49

THE BUCKY CIRCUITRY CIRCUITRY AND SOLENOID

The power which drives the grid in the direction opposite to the pull of springs comes from the solenoid which has a separate circuit for itself. In In the resting position of the grid, the solenoid contacts (SC) are chosen and the the exposure contracts (EC) are open. There There are 2 other pairs of contacts A and B which are closed through a relay which operates when the hand switch is put at “expose” position closing the contacts at “A” , applies the voltage to the solenoid (230V) through the solenoid contacts (SC) now the energized solenoid pulls the grid smoothly across adjacent to solenoid solenoid owing to the projections on the grid carriage this motion has moving effects. The grid movements begins before the radiographic exposure begins and will continue after completion of the exposure as long as the radiographer maintains pressure on hand switch. The reciprocation movements of the grid are audible characteristically. The solenoid imports a more more rapid motion to the grid grid than does not combination of spring and oil dash-spot. Utilizing the solenoid for the initial travel and the springs and oil dash spot for subsequent, phase the bucky becomes equally suitable for short and long exposures.


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To be successful, any mechanism has to meet the following list of requirements: 1) It must set the grid in motion fractionally before the radiographic exposure begins and must not cease before the exposure finishes

2) The motion must be smooth to avoid vibration of patient or cassette and its rate of movement uniform throughout the exposure.

3) The grid must move at an appropriate speed and over a sufficient distance to blur the lead strip images to an adequate extent.

4) It must not be significantly off centre in respect of x-ray tube at any movement during exposure and must be centered on the midline of the table when half the exposure is over already.

5) The mechanism of grid movement should be simple so that it can be used with rapid sequence serial film changer device also.


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3.50

COMPARISON GRIDS.

BETWEEN RECIPROCATING GRIDS AND OSCILLATING

RECIPROCATING GRID 1)

Elaborate mechanism more spare

2)

Subject greater

which

OSCILLATING GRID needs

1)

Simple effective mechanism

Film distance is generally

2)

Film distance minimum

3)

Oil dash pot unreliability chances are there due to to leak or change of viscosity of oil with change or temperature.

3)

No such problem

4)

Uneven motion and vibration may occur causing grid lines or unsharp image.

4)

No such problem

5)

Relatively longer traverse of grid and therefore increased generated cutoff in respect of any particular grid (off centring chances) more.

5)

Small traverse of grid, therefore less cutoff and decreased grid factor compared to R.G.

is

reduced

to

mechanism and no such

Time limits Stationery grids are used in tables for rapid serial angiography some Tomographic systems. In some models the bucky tray is moved by aminimotor which in supplied by a step down transformer, synchronized with the exposure timer timer switch.


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Modified X-ray Equipment For Special Purposes Purposes 1.

Portable X – ray Equipment

2.

Mobile X – ray Equipment

3.

X – ray Equipment for O.T.

4.

The Skull Unit

5.

Tomographic Equipment

6.

Mammographic Equipment

7.

Mass Miniature Radiographic Equipment Equipment

8.

X-ray Image Intensifiers

9.

Cine Fluorography Fluorography and Cine Radiography

10.

Closed Circuit Television coupling with Image Intensifier

11.

Video Tape Recorder

12.

AOT Rapid Film Changer

13.

Care and Maintenance of the X – ray Equipment

14.

General Care

15.

Maintenance of Log Book

16.

Practical precautions

17.

Brakes and locks

18.

H.T. Cables

19.

Meters and controls

20.

Tube stands and Tracks

21.

Accessory Equipment

22.

Functional Tests

23.

Failure of X-ray Tubes

24.

Failure of H.T. cables.


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Modified X-ray Equipment For Special Purposes 4.1 PORTABLE X-RAY EQUIPMENT: Any portable x-ray equipment must have the following important basic components:

1) X-ray tube 2) The tube stand 3) The H T Generator 4) The Control Unit 5) Electrical mains. In order to make the equipment simpler, lighter, lighter, less expensive and easier to move, in the the portable set, the x-ray x -ray tube and the H.T. Generator are enclosed e nclosed in one earthed metal tank filled with oil described described as the “Tank Construction” and the whole enclosure enclosure is called the “Tabe Head” The x-ray tube has a small stationery anode operating as self rectified unit connected to directly across the secondary windings of H.T.T. The x-ray tube anode has an effective focus of 1.5 mm with relatively relatively low radiographic rating. rating. The tank is filled with oil to insulate and cool the components inside it and it is vacuum sealed. Expansion bellows are provided to allow for expansion of the oil. Lead protection is included in the housing so that the leakage radiation is reduced to the desired level. There are facilities for attaching to the tube head, the beam limiting devices and telescopic metal metal centring rod.

4.1.1

The tube head is mounted on a cross arm which is carried on a vertical column. This cross arm can be moved up and down the vertical column by the method described as “Rack and Pinion”. The vertical column fits into a base which is usually in the form of “X” or “V” The base is on the castor wheels for the set to be moved about on reasonable “Tube stand” The X-ray tube can be used at a maximum height of 164 cm (65”) from floor, and a minimum height of 38 cm( 15”) from floor. The X-ray X-ray tube 0 can be tilted over 360 in the axis of cross arm (bracket holds the tube with cross arm) 0 and about 270 on its own axis. It is is dismantled dismantled when needed.

4.1.2

CONTROL UNIT The controls are embodied in a small box that can be hung on the vertical column or on the Control desk or kept on a supporting tray. The low tension cables coming from the tube head has at its other end a plug which fits into a socket on the control unit. Another socket on the control unit take the plug of a thinner caliber which has a timer timer switch at its further end. A third lead from the control unit is the mains cable which carries at its further end a plug which fits into the wall socket on the control control box.


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The following features are are noted: 1) The mains ON/OFF switch 2) Voltmeter indicating the line voltage 3) Mains voltage compensator (autotransformer) (autotransformer) knob which can be rotated in steps to bring the line voltage to 230 v 4) KV Control knob 5) mA control knob 6) Milli ammeter which records the tube current 7) A change switch for radiography /Fluoroscopy 8) Filament press button switch to select the MA Range TUBE STAND AND CONTROL;


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4.1.3 THE mA CONTROL The tube current control knob allows x-ray tube current to be adjusted to the value required for radiography or fluoroscopy. This knob can be set at selected value of10 mA,15 mA,15 mA and 20 mA and so on depending on the type of equipment. 4.1.4 THE KV SELECTION CONTROL:

This knob is manually adjusted over a series of setting stubs. stubs. The control gives selections of KV over range which is about 46 to88 KVP. It is noted that KV knob and mA knob connections are interlocked. 4.1.5 THE TIMER SWITCH:

This is a hand switch which can has a scale marked with time intervals in seconds and fractions there of from zero to 10 seconds on its dial. Usually and is divided into 4 parts (0.25 sec) there is a pointer which moves on the dial to show the times switch. This initiates the exposure and the pointer begins its return journey to the zero position. There is an additional button on the dial to reduce the time set by mistake to the correct position which, when depressed brings the pointer to the correct level without allowing the exposure to occur. 4.1.6 FLUOROSCOPIC SETTINGS: A portable unit may be often provided with fluoroscopic attachment. Fluoroscopy may be done at 4MA upto 88 KVP, when the times switch is disconnected and its place a for switch is connected from the point of view of radiation safety and in the interest of the equipment which is of low power, it is better to forget that the simple portable set be used for fluoroscopy. 4.1.7 RADIOGARAPHY OUTPUPTAND MAINS REQUIREMENTS:

The portable set operated at 230 V may draw 12 amps when operated at 200 V mains supply, it draws 14 amps of current, when only 5 amps power supply is available at a domestic mains the set when operated at 10mA draws 8 amps current for a very short period of time, so the fuse may not blow off. But If operated in15-20 mA range the fuse will certainly blow off in a 5 amps supply. The portable set is better operated at any mA range if 15 amps supply is available. Some of these sets are even provided with over load circuit breakers for safety. 4.2 MOBILE X-RAY EQUIPMENT:

The term Mobile X-ray Equipment covers a side range of apparatus, with low powered machines at 10-15 mA at 40-90 KVP in one extreme and up 300 mA at 125 KVP 150 mA at 95 KVP,etc. The simplest mobile equipment as similar to that of the portable set already described having the essential components of tune stand, tank type of tube head including the HT circuit and control. Unit, cables connecting the timer and the mains power supply. Since this type has been already described only the powered mobile x-ray equipment is dealt with in the following paragraphs. A high powered mobile x-ray unit Fundamentals Of X-Ray Equipments

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4.2.1 X-RAY TUBE: Is dual focus/single focus rotating or stationary anode tube. The focal spot combinations of 1.0 mm for fine focus and 2.0 mm for broad focus. 4.2.2 TUBE STAND: The base of unit is 63 cm (25”). It has 2big wheels at the back and 2 castor wheel at the front and a steel bumper bar at the sides and in the front. A strong vertical column mounted on the base supports the cross arm which carries the the x-ray tube. The cross arm arm has la la telescopic extension which allows the tube to be positioned at a distance of 106 cm(42”) from the centre of the vertical supporting column can be raised is 210 cm(82”) from the ground. TO TOP COLUMN


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The tube can be brought down to the minimum height from from the floor of 76 cm(30”) . for transport purposes the x-ray tube camber brought down the vertical column and turned round with the cross arm over the base so so that the unit becomes compact, and more easily maneuvered. Some mobile units are provided with motor drive to take them them up the ramps being energies by 12 volts batteries. 4.2.3 H.T GENERATOR: gives a maximum out put of 300 mA 125KVP. The H.T generator includes full wave rectification provided by means of selenium rectifiers. rectifiers. This gives greater efficiency efficiency and results tin lower main current being drawn. The H.T generator, its rectifiers rectifiers and the filament transformer for the x-ray tube are all enclosed in one oil filled earthed steel tank. This tank has a fibre glass cover and is mounted on the base of the unit immediately behind the vertical column and the batteries for the motor drive H.T tank is connected to x-ray tube by means of H.T cables 4.2.4 CONTROL UNIT:

This is located under a hinged lid behind ther H.T tank on the unit. The features are listed as follows: 1) The lever selected the forward and reverse for motor driving of the unit 2) An ON /OFF switch of the drive unit 3) Indicator lamp showing switch on 4) mA selector which allows selections of 6 different tube currents. 5) Overload warning light 6) A timer with a range of 0.02 sec to 5 sec 7) mAs meter and mA meter (working before and during exposure) 8) buttons to control up and down movement of x-ray tube. 9) Mains switch off and on 10) KV control in 40 steps 11) Bucky control switch (in one equipments) 12) Pre-reading Kv. Meter scaled from 0 to 130 KVP 13) Lamp showing x-ray exposure taking pulse 14) Line resistance selector switch 15) A hand exposure button switch with 6 feet long wire 16) cable connecting the unit to the power supply mains.

4.2.5 RADIOGRAPHY :

The exposure for the radiography is done by means of a small hand timer with2 scales. These have the times times of the other scale.


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4.3 X-RAY EQUIPMENT FOR OPERATION THEATHRES:

When x-ray equipments are used in 0 to 3 important hazards or a to be considered: 1) Risk of infection to patient from the equipment u sed else where also 2) Risk of explosion in an atmosphere atmosphere flu of anesthetic gasform as spark from the x-ray x-ray set. 3) The radiation hazard. The risk of infection is avoided by shielding with lids, lo those parts which can collect dust and cleaning of the whole of x-ray with antiseptics including the cables. The explosion risk is avoided by the use of x-ray control unit ( which contain the sparking relays and switches) kept in the ante-room and only thx-ray tube with tube stand and H.T Generator is allowed to be inside the inside the theatre. The cables connecting the x-ray set in theatre and control in the anteroom are well insulated. Another source of explosion risk being the static electricity produced when the wheels move over the floor of theatre. A MOBILE IMAGE INTENSIFIER & TELEVISION MONITOR


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Theatre twin head units has 2 tube head (tank construction type) mounted on its own cross arm one above the other on a single single vertical support column. This This equipment is intended to be used on occasions when both AP and lateral projection are to be taken without moving the equipment as in the case of a SP nailing operation for the fracture of the neck of femur. A switch control in the panel selectively energizes either tube and the 2 views are exposed one after the other without disturbing the equipment.

4.3.1 RADIOGRAPHIC OUTPUT AND MAINS REQUIREMENT

In case of a mobile 300mA unit the maximum line resistance that can be tolerated is 0.34 ohm. When set to operate on 240 V supply. On a lower power supply voltage, the same unit gives rise to bigger primary current and hence bigger voltage drop on the same resistance. resistance. Therefore, this unit provides on its control panel, the special switch which is called the line resistance selector which can adjust the mains resistance. The maximum radiographic factors in this mobile set with draw very very heavy currents from the mains A 15 amps mains supply is not enough, A 30 amps supply point is needed but if resistance in the mains mains supply is adequately low, this set can be used occasionally for minimum number of exposures with 15 amp special type of power sockets.

4.3.2. MOBILE UNIT INDEPENDENT OF MAINS SUPPLY:

These are called the energy storage units because, the x-ray sets have the needed electrical energy for its tubes x-ray exposure stored in some form or the other. There are 2 forms, one in which x-ray tube obtains energy from the discharge of a capacitor. The second type tube obtain energy from the electrical batteries.

4.4.

THE SKULL UNIT

4.4.1. GENERAL FEATURES The skull unit is characterized by following parts 1) A fixed column which supports the object table and x-ray tube together 2) An object table which is essentially a potter bucky tray. This is counter balanced. The distance between between the object table table and x –ray tube is constant and has a constant anode tube distance of 90 cm or 36” 3) A double semicircular arches at either end of which are a) the x-ray tube b) a counter weight for the tube. The object table and their arches are mounted on the column by means of a single carriage.


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100 X- RAY EQUIPMENT FOR RADIOGRAPHERS

4.4.2. ATIENTS’ COUCH:

Associated with the skull table is a trolley or couch on which the patient can be lying with his head on the skull table. Some times this couch can be raised up and lowered down hydraulically.

Fundamentals Of X-Ray Equipments 100


SKULL TABLE VARIOUS VARIOUS ANGULAR ANGULAR MOVEMENTS

4.4.3. MOBILE IMAGE INTENSIFIER (I.I):

During in surgical procedure one way of saving time is to employ a mobile unit for fluoroscopy with image intensifier. This reduces the number of radiographs to be taken, thus the time of processing them and also reduces reduces the radiation dose. Further by internal diaphragm the x-ray beam is carefully limited to the small required field size of 13 cm on 11 tube a TV link may be added to to the equipment so that the x-ray image may be viewed on the monitor screen by more than one person at a given time during surgery. X-RAY TUBE HEAD AND II of the unit. The x-ray source is in the form of a fixed anode xray tube with an effective focus of about 1.8mm . This operates in at 0.3 to 5 mA for Fluoroscopy and 20-25 mA for radiography. The maximum tube KVP is 70 – 90 range depending upon the mA being used. X-ray tube is self rectified rectified and is enclosed with its its H.T. generator in a single tank construction, the tube stand is mounted mounted on one end of C shaped arm. On the other end of C arm is the 1.1 The C arm is supported on a cross arm which extends from a carriage at the top of the vertical column, which is mounted on the base of the unit. The vertical column can be varied in height. The base is mounted on wheels and also the base carries carries the control panel and control circuits. Fundamentals Of X-Ray Equipments 101


4.4.4. TUBE CURRNET (mA) SELECTION:

The most selector knobs allow selection of 6 different stages of 25 mA, 50mA, 100mA on fine focus and 150mA, 200mA on broad focus. The tube current is interlocked with KV and time so that warning of overload is given to prevent selection beyond the rating. 4.4.5. KV SELECTION

Is by a knob manually controlled. This knob rates 5 times to give 40 steps of selection upto a maximum of 125 KVP. Before the KV is selected, mA range has to be selected this KV is selected is shown on the pre-reading KV meter. 4.4.6. TIMER :

The exposure time is selected by means means of a manual manual control on the the control panel. Timer Timer is an electronic type and has a range from from 0.02 sec upto upto 5 seconds. To make another exposure of same duration, it need not be reset as this this is an electronic timer. 4.4.7. MOVEMENTS OF SKULL TABLE OF OF X-RAY TUBE:

The basic concepts of the skull table table design are apparatus capable of moving the patients head to suit certain angulation of x-ray beam. Secondly, to provide the x-ray beam to be constantly centered upon the grid and film. The unit can be adjusted in the following ways with ease: 1) The whole device of object-table and counterweight containing arches and tube arches can be moved enblock up and down the supporting vertical column and fixed in required position. 0

2) The whole device can be rotated through 360 vertical and associated positions.

permitting the object table to be in

3) The x-ray tube and object table can be moved moved independently. While the table remains stationary, the tube and its arch can be rotated round it. This permits angulation of the xray beam in the saggital line of the patient (eg. Towne’s View.)

We can have: a. Angulation of x-ray tube only b. Tilting of object table only table plus some some tilt of x-ray tube. c. Combination of some tilt on the table

4)

The x-ray tube can swing on its arch in a direction along the arch. The total range of 0 0 0 movement is about 120 This comprises 30 towards the column and 90 away from column. Fundamentals Of X-Ray Equipments 102


5)

The x-ray tube can be tilted independently in its own axis.

6)

The bucky is on a pivot and can be rotated through 360 , so that the beam inclinations can be centered to the centre of the grid with out stands.

4.4.8

ACCESSORIES

0

Like head immobilizing devices, beam limiting and beam centering devices also are available. Spring loaded hand operated cassette changes can be combined with skull table (after removing bucky mechanism mechanism of AP position) and without disturbing it for lateral projection. 4.4.9

TOMOGRAPHIC EQUIPMENT

A. TOMOGRAN consists of: 1) the sharp image of selected layer in the body and 2) unsharp densities due to movement blur in layers above and below selected layer.

TOMOGRAPHY is performed as follows 1) The x-ray tube and the film are moved through equal and opposite excursion 2) There is a fulcrum fulcrum about which this this above movement must must revolve. The fulcrum fulcrum itself does not move 3) The fulcrum determines the layer depth in which the radiography is done.

TOMOGRAPHIC equipment is so devised that 1) the height of the fulcrum layer above the film can be pre-selected between 0-20cm 2) the thickness of the fulcrum layer can be altered within limits by the use of different angles of tubes 3) the movement of the tube can be linear (simple apparatus) or multidirectional (complex apparatus) 4) speed of travel is variable. If Tomography is to be successful, it is important that the film remains parallel with the selected layer in the body and the image magnification is constant throughout the movements of the tube and film.



Equipment for Tomography may may be of 2 categories: 1) Tomographic accessory attachments

to a standard radiographic table and the tube

support 2) Special tomographic table per-se



4.5.2 TOMOGRAPHIC ATTACHMENTS

The components of attachments to standard x-ray unit are 1) a linkage mechanism 2)a pivot unit 3) a mechanical drive 4) a drive control



4.5.2 LINKAGE MECHANISM: The link assembly is a long telescopic steel rod which couples together the x-ray tube and the bucky carriage by means of clamps and locking handles. The end of the rod which is attached to the tube carriage, may be so designed that when the rod is in position it is impossible to tighten that lock, which prevents the rotation of the tube. this ensures that the tube tube is free to rotate rotate on its axis, at the extremities of its travel. The radiographer however must release the other locks locks such a bucky tray lick and floor floor brakes of tybe column. The locks for transcribe transcribe movement of tune and ver5tical movement movement of the tube must must be firmly licked. licked. The anode –film-distance –film-distance is adjustable between 36”-42”(92-107cm). 4.5.3.THE PIVOT UNIT

it is a Turret like stature called the fulcrum tower tower about 12” to 15” (30-38cm) high. This is fitted to the edge of the x-ray table on the tube stand side. The linkage arm passes through through a pivoting sleeve on the side of the tower. The functions of this unit is to provide fulcrum for the opposite movements of x-ray tube and bucky tray and also the means for the height of the pivot point by a worm-screw, which can be hand – turned by a knob from0 cm to 20 lkin step of 1cm each on the scale, which is indicated by the pointer provided. The fulcrum tower includes a switch assembly, which effects x-ray exposure by way of 2 contacts at the correct juncture. 4.5.2 THE MECHANICAL DRIVE

Is provide by a small motor (for purpose of radiation safety it is not hand moved). The base of tube stand is attached to the motor by a lever-mechanism. DRIVE CONTOL is a separate wallmounted box; this has switches which control 1) the selection of tube speed of travel 2) selection of angle of exposure 3) the trial trial runs o the apparatus (with (with – out x-ray exposure ). ). Variable 3 speeds of tube travel travel may be available. There may be anlgles of exposure of 22 and 44 or a range of angulations in5 steps from 30 to 60. 4.5.5 SPECIALLISED TOMOGRAPHY TABLES:

The outstanding characteristics of specialized tables are : 1) the x-ray tube is permanently swinging or giving accurate linear movements 2) this table can be turned to vertical, thus permits Tomography to be made with the patient is in erect posture. 3) Many of the movements of this table tab le are motorized.

4.6. MAMMOGRAPHIC EQUIPMENT: The demands of a mammography examination are peculiar in that : 1) the part is entirely a soft tissue, whose normal and abnormal tissue absorb x-rays almost the same degree of translucency 2) Soft radiation of longer wavelength are required and hence low KV must be used 3) The film used for recording must be able to resolve fine details, having film emulsion of fine grain an must be used with our intensifying screen. Fundamentals Of X-Ray Equipments 106


4) In addition to low KV we need mAs to be used. The exposure times are dismayingly long 5) Careful immobilization of the breast is needed for this time con suming exposure period 6) Some form or compression of the breast is necessary Mammography can be performed by 1) modifying the general radiographic equipment available in any x-ray x-ray Dept. 2) specially designed Mammographic equipment. 4.6.1 MODIFIED GENERAL EQUIPMENT

THE X-RAY TUBE: The aim being to maintain x-ray of long wavelengths 1) the added filtration of Aluminum has to be removed 2) the light beam diaphragm containing collimator unit has to be removed 3) A long cone replaces the collimator unit which is used to limit the field size covering the entire breast from nipple to anterior chest wall THE PATIENT AND FILM SUPPORT: A variable height support is required required on the table which supports the film and also in the seat seat on which the film must be adjusted to suit this purpose. In the THE GENERATOR the modification required is the H.T generator to be able to provide a selection of low KV range of 20 to 40 KV. 4.6.2 SPECIALISED MAMMOGRAPHIC EQUIPMENT

These are considered in main parts 1) the x-ray x-ray tube 2) Mammographic stands 3) complete mammographic units. 4.6.3. X-RAY TUBNE FROM MAMMOGRAPHY :

The Mammorgraphic tube must have the following :

1) Close electrodes spacing, to enable the cathode to be used at lower levels of heating of he filament for a given mA. 2) The anode is made of molybdenum which gives a narrow band of longer wave length xrays which are intense. 3) The window of the x-ray tube must be made of thin beryllium glass, through which the xrays pas out of the tube. tube. This helps in lesser inherent filtration. 4) Instead the tube has a molybdenum filter which removes x-rays of shorter wavelengths, at the same time allowing the rays of longer wavelengths to pass through 5) Focal spot sizes inmammography inmammography (MGT) tubes vary from 0.6 mm to 1.0mm or 2 mm. mm. a boarder focus helps to reduce the risk of movement unsharphess, as it allows shorter period of exposure and also increases the penumbra.



6) The tube unit is fitted with long cone which is D shaped and is adopted well for the closer positioning of tube unit against the patient’s breast and chest wall 7) The tube insert has a rotation anode of Molybdenum filter, close electrode spacing a beryllium window and a molybdenum filter. Within the window, there is a collimator close to the focus which is intended to cut off the radiation arising form parts other than that of focal spot. 8) The x-ray tube is so positioned that the flat cathode end is towards patients face which allows one side of beam radiation having an edge to the chest wall.



4.6.4 MAMMOGRAPHIC STANDS

This stand has its special special x-ray tube mounted on tube carriage. This tube carriage hold x-ray tube rigidly coupled and aligned to a little table, which serves as a hold for the film as well as support to the the breast under examination. Some stands possess additional radiolucent plate which compresses breast between it and the film plat. Mammographic stands are designed to be flexible in operation and easily manovrable, so that the patient may be examined in standing, sitting of lying. The required projection may be obtained conveniently and is also reproducible.

The movement of the tube tube unit and the film film holder/breast, holder/breast, support are

controlled by the electromagnetic brakes which hold them firmly. 4.6.5.THE GENERATOR:

is some times an integral part of the mommographic unit or the mammograpic x-ray tube are connected to HTG of another x-ray unit already in use the department.

4.6.6. COMPLETE MAMMOGARPHIC UNITS:

this is a self contained unit designed for only mammography. mammography. They are often mobile, enabling them to be used for a patient lying on another x-ray couch. this or the fixed fixed version of the unit, comprises of the following features.



1) The breast support and the film holder are designed so that it can hold the special xeroradiography cassette on the film holder as an alternative to the conventional nonscreen cassette. 2) A breast compressor in the form of a plate or as an integral part in the beam limiting cone of peculiar ‘D’ shaped 3) some breast cones 4) x-ray tube (specially designed for mammography ) 5) A full range of movements for the tube assembly and the breast-support breast-support the film holder, so threat the required projections may may be conveniently obtained and the patient can be examined easily in erect or laying down position.



6) A tube support by means of which the tube unit is rigidly coupled an aligned to the breast support and the film holder 7) A special HT generator for the unit which will provide a range of low KV voltages (2040 KVP) 8) An automatic timer control for exposure 9) A seat for the patient which is adjustable up and down and can be moved or rotated 10) A control panel behind the lead protective screen with a lead glass window for the radiographer to look through during exposure. 4.7 MASS MINIATURE RADIOGRAPHY EQUIPMENT (M.M.R)

M.M.R is presently applied to the examination of chest of a large number people in surveys for excluding tuberculosis tuberculosis in lungs. the general principal is recording of the fluoroscopic fluoroscopic image of the chest in small films, 70X 70mmor 100X100mm size roll film orcut films loaded in a magazine. Patients are examined in erect posture on postero anterior view. The apparatus for MMR may be considered in3 parts : the; ;x-ray power unit the camera unit and the control table . Figure



4.7.1 THE POWER UNIT

Consists of a light pyramid shaped funnel fitted at the smaller end with the camera and at the larger end with with the fluorescent fluorescent scream, protective lead glass and radiographic grid. The funnel also carries the identification deice., the camera is fitted with a fluoride coated 2” lens of focal length of 1.5 m \. The coating of the surfaces of the lens eliminates eliminates internal reflection and thus ensures maximum definition, contrast and speed. Figure:



Focusing of the lens is precision adjustment. Schmidt helm mirror optics uses the a spherical correction plate placed at the centre of curvature of the mirror, producing high definition (Bouwer’s concentric mirror system in which the correction plate is replaced by a low power and power and readily reproducible meniscus correction lens giving high definition an covering an unlimited field). field). Bouwer’s cone lens addition addition increases the resolving resolving power of the concentric mirror system to give high definition . The 70mm odelca unit accommodates both cut films and roll films ( the cut films are loaded in separator cassette) the roll film being loaded loaded in a 50 exposure roll film magazine which may be hand rotated or motorized for consecutive exposures. But a 100mm. odelca accommodates only cut films of 100X 100mm size. 4.7.3 THE FLUFOSCOPIC-SCREEN

of yellow green or blue fluorescence type is 16” square so that it may include the whole of the largest chests. The focus screen distance being 36” to 27” in different different units., there may be some enlargement of the screen image. The tune diaphragm is adjusted to the x-ray beam to the area of this screen. screen. There is a lead glass protection on the camera side of the screen which absorbs about 10% of the fluorescent screen illumination).there is a stationary grid to reduce the scattered radiation. 4.7.4 IDENTIFICATION:

Of a chest radiographs is achieved by photographic print at the lower border of the radiograph the serial number (on the patients requisition form) on the record card being placed in the slot provided for that purpose, in the apparatus. ap paratus. Correct alignment between xray tube and screen is important and may be checked by means of an optical centering device. Most units have readily connected system of combined movement of the screen and x-ray tube in which the patient is moved up an down as required by the movement of a pedestal rest by a motor. 4.7.5 THE CONTROL TABLE :

Is so designed that, when set for the appropriate exposure, the single control switch automatically brings into operation correct sequence, the rotor of the x-ray tube, the exposure number on the record card, the exposure itself and final, the winding on the film in the magazine. Meters and light indicators warn the operators from radiation. Radiation protection is afforded by 2 screens of o f lead lining and lead glass windows on either side of the patient. These lead unit screens together prevent scattered radiation from reaching others in the room including the technical staff.



4.7.6. EXPOSRE TECHNIQUE :

It is necessary to increase the routine large film Teleradiography factors by 2 to 3 times in the M.M.R as less light reaches the fluorescent screen than the I.S of x-ray cassette. The FFD is 36” or less in M.M.R and as the grid is also present in M.M.R M.M.R the KV is raised by 20KV than for Teletadiography or a similar increase in mAs alternatively. As in some departments the M.M.R x-ray examination done in a day may exceed 100 or more, there is need for standardization of exposure to produce uniformity in results and hence this is achieved by use of a photo timer, which automatically adjusts the exposure to the density of the subject, using 3 alternative KV settings coupled with mA correspondingly for coverage of large and small small patients. The KV ranges from 60-65 KV at 40mAs to 80-85 KV 20mAs. 4.7.7 POSITONING OF THE PATIENT:

Is similar to that teleradiography with the chin resting on the chin rest and chest closely touching the screen and shoulders rolled towards, touc hing the screen and wrists on the waists. 4.7.8 VIEWING OF FILMS:

There are standardized tanks specially meant for roll films on the spiral frame. For toll films development is for 7mts at 68 f., in in the dark room with red safelight. The rinsing and fixing are done in the other 2 circular tanks provided for that purpose. 4.7.9 VIEWING OF FILM :

A suitable projecting illuminator like Heliocontrast is well suited for viewing MMR films. Alternatively, we can use a magnifying lens fixed above a wooedn box containing 2 feet tube light. The film is kept between the the lens and illuminating source and viewed viewed by adjusting the magnifying lens accordingly. 4.8 X-RAY IMAGE INTENSIFIRES (I.I)

The image intensifiers have revolutionized the fluoroscopuy as its image is bright enough for scotopic visionand is also suitable enough for coupling to the cine camera, T.V. video camera or spot film camera. The power of brightening of the fluoroscopic images is called the image intensification. The th benefits of image intensifiers are 1) potentially lower rendition doses about 1/1000 of fluoroscopy to patient and operator, because very low tube current (mA) is used 2) can be used for prolonged periods 3) the image is far superior.



IMAGE INTENSIFIERTUBE

4.8.1 COMPONENTS OF I.I

The is an electronic vacuum tube made of glass varying screams screams of 5”, 7”, 13”, 18”,& 23” diameter. 1) Input phosphor and photocathode 2) Electrostatic focusing lens 3) Accelerating anode 4) Output phosphor. After the x-ray beam passes through the patient, it enters I.I tube via the input phosphor screen which absorbs the x-ray photons and converts them in to light photons. The light photons strike the photocathode causing it to emit photo electrons. These photoelectrons are drawn away from Photocathode by appluy8ing high PD. Between it and the accelerating anode. The electron flown from photocathode to accelerating anode are focused by an electrostatic lens which guides them to output phosphor screen the electrons, that strike the output screen emit the light photons which gives the image for the human eye to observe. There is no geometrical geometrical distortion during the above phenomenon.



4.8.2 INPUT FLUORESCENT SCREEN :

Is made up in the present present generation I.I tube of caesium iodide crystals. This is usually 5 to 9 inches in diameter. 4.8.3 PHOTOCATHODE:

Is comprised comprised of photo emission metal ( antimony, and caesium, potassium, potassium, or sodium compound). Apart from its photo emissive emissive action it also serves as the cathode of I.I. tube this this is kept at ground potential and is adjacent to input input phosphor screen. screen. It emits electrons (15-20 electrons for every 100 light photons).



AN IMAGE ORTHICON CAMERA TUBE

4.8.2 ELECTROSTATIC FOCUSING LENS:

Is made up of series of positively charged electrodes, that are plated on the inner surface surface of the glass envelope. These electrodes focus the the electron beam from the photocathode to the output phosphor screen. The image on the output phosphor is reduced in size. Which is why it is brighter.

4.8.5 ACCELETATING ANODES:

Are located in the neck of I.I. tube. Its function is to draw electrons from photocathode and accelerate them to the output screen. The anode ahs appositive potential of 25KV which accelerates electrons which tremendous velocity.



ELECTRON – OPTICAL MAGNIFICTION

4.8.6 OUTPUT PHOSPHOR SCREEN:

Is made of silver activated zinc sulphide but the crystal seize and layer thickness is so reduced to maintain resolution power of the minified image. Since the electrons are greatly accelerated, they emit more light photons from the output screen. They are light photons increased to 50 folds. The output screen is about 1” in diameter. A series of mirrors or a T.V camera allows the fluoroscopist observe their bright minified fluoroscopic image. The glass tube of I.I is enclosed in a lead lined metal container in order to prevent stray irradiation and for purposes of prevention of mechanical of magnetic interference. Moisture an dust proof cove is mounted on the C arm of the fluoroscope. There is also a lead glass window window over the outputs phosphor which absorbs any further radiation after the passage through the I.I tube. An adopter plate allows the I.I to be fitted to the fluoroscopy carriage in the place of usual fluoroscope. Screen which is removed and kept aside. The I.I is balanced by means of a ceiling suspension bracket and is easier to handle.



ELECTRON-OPTICAL MAGNIFICATION

4.8.7 THE VIEWING SYSTEM:

Is optically coupled to I.I tube my means of a tandem lens. The output phosphor image intensifier has a diameter diameter of 1.25 -2.5.cm which is to small to appreciate details. This image is either directly viewed through a series of lenses and mirrors or indirectly through a closed circuit 0 T.V. A movable mirror which is mounted at 45 angle between tandem lenses allows the operator to switch on the viewing systems and recording system. With a semi-transparent mirror the image can be monitored during during cine fluorography. Thus the image is magnified magnified from 1.25 cm to the practical size for viewing and recording, through the objective lens.(diagram on page no:163)

4.8.9 BRIGHTNESS GAIN:

Of I.I consists of 2 factor 1) Minification gain and 2)flux gain. The minification gain is 81 (9x9). The flux gain is s50. Total brightness gain =81x50000=4050.



4.8.10 IMAGE QUALITY :

The brighter image (of 4050times ) has several benefits. No dark adaptation is necessary. Fluoroscopic rooms can afford to have the room lighting on, which relieves the patient apprehension to the darkness. Fluorography is possible with this lighting. The TV coupling helps in the teaching for the student fluoroscopists.



4.8.11.CONTRAST:

Is diminished in II due to 2 reasons 1) due to back ground fog by unabsorbed transmitted photons form input screen 2) due to retrograde light flow from output screen that activates the photocathode, which causes some fog.

4. 8.12. RESOULTION : Of image in I.I tubes 1-2 line pairs/mm. the resolution of caesium iodide I.I tubes is 412 line pairs/mm. 4.8.13. DISTORTION :

In the centre of I.I tube there is better resolution, a brighter image and less geometric distortion. In large I.I tubes, the periphery has less resolution, less bright image and more geometric distortion. The fall off off brightness at the periphery periphery is called ‘ VINETTING VINETTING ‘. So the smaller smaller the image of I.IO tube the better is the quality of image than the large I.I unit.

4.9 CINE FLUROGRAPHY AND CINE RADIOGRAPHY:



Cine fluro-radiography is the process of recording the fluoroscopic images in the cine film. Clinical application of cine fluorography came into vogue in1950 after the invention of I.I whose image was bright enough and small enough to be recorded by cine camera coupling to I.I Two film sizes 16mm and 35mm are currently in use. 95% of cine studies involve study (angiocardiography ) heart and great vessels. The basic components of cine fluorography systems are 1. Cine camera 2. Image intensifier 3. Opticalsystem4. X-ray photon mage5. Radio graphic graphic equipment 6. cine film 7. film film processor 8. Film projector.

4.9.1 CINE CAMERA: Basic components of cine camera are: a lens, light diaphragm, shutter, aperture, pressure plat, pull down-arm and film transport mechanism. The x-ray exposures and shutter openings are synchronized by electrical signal from from drive motor. The framing frequency of the the number of frames/seconds is usually 7 ½ , 15,30,60 or120 frames per second.



4.9.2 IMAGE INTENSIFIER:

Caesium iodide tubes are used as they give better resolution and better contrast. The optimum size image intensifier for cine fluorography is 6 inches (15 to 17) cm for coronary angiography. Larger multiple mode I.I are used for adult ad ult angiocardiography. 4.9.3. OPTICAL SYSTEM: 0

conveys the image FROM I.I through tandem lenses separated by beam splitting mirror at 45 TO THE CINE FILM. 4.9.4 X-RAY PHOTON IMAGE:

Is an important important requisite for optimum cine fluorography. The is the image that exists exists in space after the x-ray beam has traversed the patient and before the image reaches the I.I. 4.9.5. X-RAY APPARATUS :

Many of the angipocardiographic radiology departments are designed for serial filming and cine fluorography. The energy demands for cine are generally less than those for serial filming. filming. In the apparatus, we have to consider mainly the generator and the x-ray tube. THE GENERATOR must be 3 phase 12 pulse constant potential one. The current output (mA) rating rating must be 3 times the cine pulse output to minimize minimize the voltage fluctuations. A 700 to 1000 mA, 100 KVP generator is optimum, if the transformer design ensures stable voltages and currents during pulsing, then a 500 mA generator will suffice. Fundamentals Of X-Ray Equipments 123


4.9.6 THE X-RAY TUBE : Cine fluorography has heat over. A tube with with following characteristics recommended. recommended. Focal 0 0. spot size 0.6mm (micro focus), Heat rating 60KW, Ano de diameter 100mm, target angle 6 -12 cooling circulatory liquid and a rotational velocity 10,000RPM. 4.9.6 CINE FILM:

The types of cine films are acetate based and polyester based. The most important qualities of cine film to be considered ar5e the speed and contrast. 4.9.8. PROCESSING : Automatic processing of cine films are preferred if they are designed for motion picture processing. These are very expensive. Processing should be do by skilled personnel only. 4.9.9 CINE PROJECTS :

There are a few projects that are desirable for cine fluorographic age review (Tage Anno and Vanguard) . There are 3 very important mechanisms mechanisms for proper functioning. functioning. 1) a film transport mechanism 2)a light source and condenser optics 3) an imaging optics.



4.10.CLOSED CIRCUIT TELEVISION (C.C.T.V) COUPLING WITH IMAGE IN TENSIFIER:

The display of fluoroscopic image on the television monitor has become possible after the invention of I.I only. Its small output phosphor simplifies the optical cooling, and a brightness again of 3000 to6000 times produces a strong strong video – signal to be picked up by a video camera.



The Videocon camera is now very much improvised for coupling with I. I.I closed circuit T.V. monitoring. This C.C.T.V coupling with I.I consists of the following components 1) Videocon Camera 2)Camera control unit 3) TV monitor. 4.10.1 CAMERA :

In this, a lens system conveys the fluoroscopic image from the phosphor of the I.I to the TV camera where it is converted into a series of electric pulses called ‘Video signals’ which are transmitted through cables to the the camera control unit. In this unit these signals signals are amplified and then forwarded through another cable to the TV monitor. The TV monitor converts the video signals back into original image for direct viewing. This camera is about 5 inches in diameter diameter and 9 inches long. The most important part of this is the “Videocon Tube”. This tube is surrounded by two pairs coils called electro magnetic magnetic focusing coil and the electro magnetic static deflecting coil. 4.10.2 CAMERA CONTROL (C.C.U) :

the image from the I.I is focused on the target assembly which consists of 3 layers 1) a glass face plate 2) a signal plate 3) Target. A VIDICON VAMERA TUBE



The electron beam from cathode scans the electric image stored on the target and dis-charges tiny globules which act as tiny capacitors. At the instance of discharge, a current current flows thorough the conductive signals plates and this from the video signals. A series of video pulses are originate from signal plate. The camera controller unit takes the the task of reassembling these pulses back to visible image with the help of a television monitor . it (C.C.U 0 amplifies the video signals, regulates the coils and synchronies the video signals between the TV camera and the TV monitor. Plulmbiucon camera is a recent improvement of videocon camera tube employing different conductor layers on target (leadoxide) it gives uniform picture. Isoscon camera tube is modification of orthicon tune in which the image is free from noise. 4.10.3. T.V MONITOR:

The contains a picture tube and controls for regulating the brightness and contrasts of into image. This is the last in V chain system. The cesium picture tube contains electron gun, control grid, anode, focusing coils and deflecting coils, these coils control the electron electron beam inexact synchrony with the camera tube. The brightness of the individual dots in the picture is regulated by control grid. It receives the video signal from TV camera ca mera and uses this signal to regulate the number of electrons in the electron beam.

For production of a bright area in TV pictures more number of electrons are required, for dark area production it cuts off electron flow almost almost completely. The anode is plated into the inside Fundamentals Of X-Ray Equipments 127


surface of the picture tube near the fluorescent fluorescent screen. It carries a much higher positive positive potential of 10000 V than the camera tube (250V. so it accelerates the electron beam to much higher velocity. Many secondary electrons that are set free on impact with screen screen are attracted to the anode and conducted out of the picture tube. 4.10.4 TV SCANNIG:

TV image is stored as an electrical image onthe target which is scanned along 525 lines by an arrow electron beam 30 times a second. Each scan of the the entire target is called a FRAME. So, there are 30 frames per second. This is perceived by the the eye in its owns low fashion ( in the same way as we see a camera in motion picture) and so flicker is detected by our eye because of an electronic trick called interlaced horizontal scanning. Band – pass of band – Edith is the frequency image from lowest ot highest in the (image) video system.

(VTR)(VCR)(VDR): 4.11 VIDEO TAPE RECORDER (VTR)(VCR)(VDR): The same unit acts as recorder unit and also as play back unit. It transmit it’s the signals signals to one or several TV monitors through cables in a close cirvu9it cirvu9it TV system. There are 3 essential components, besides the electrical circuitry. These are 1)Magnetic tape 2) A recording cum reading head (writing head ) 3) a tape transport system.

4.11.1. The writing head convert to an electrical signal into a fluctuating magnetic field for recording. This same head converts the the magnetic field signal into an electrical signal for replay. The drive spindle moves the tape past he writing head at constant velocity (speed (speed ). The tape is kept in contact with the writing head always both during play back and recording. The head is similar to a transformer in that it consists of a magnetic core such as an iron nickel alloy wrapped with2 coils of wire. It differs in that a narrow segment segment or gap is cut from the core, secondly 2 coils are wired together so that their magnetic fields reinforce each other. The magnetic field Fundamentals Of X-Ray Equipments 128


extends out beyond thegap in the writing head. This extend magnetic field is the critical portion that interacts withj the magnetic tape. VIDEO WIRTING HEAD

4.11.2 The magnetic layer of video tape is composed of oxides of magnetic materials. The molecule of this material behave like small bar magnets magnets or DIPOLES which aligns itself in the magnetic field like the needle or a compass. They are randomly arranged in an unrecorded tape. As the recording takes place their alignment changes to coincide with the magnetic field impressed upon them while they pass the gap. MAGNETIC FIELD



4.11.3 When the video signal is negative, the dipoles are aligned to the left and when the signal is positive, the dipoles are aligned to right. Play back is exactly the reverse reverse of the recording process except ex cept that the magnetic dipoles dipo les are not aligned in the play back. b ack. The partially aligned magnetic dipoles have a magnetic field of their own. As the field moves moves past the gap in the writing head, it indices a magnetic field in the wire coils. This is the video signal that is forward transmitted to the display T.V. monitor. 4.12.AOT RAPID FILM CHANGER: Is an automatic rapid film changer changing film as rapidly as 6/sec or as slowly as I film for5 sec Two AOT changers may be coupled to work synchronously or out ot phase for simultaneous AP and lateral views. AOT changer accepts films films of conventional sizes of 12” x 10” or 14” x14”. The AOT changer has following parts 1)loading magazine for unexposed films 2) an exposure section where a film is held stationary between intensifying screens while exposure is being made. 3) A receiving cassette for exposed films. 4) A mechanism for transport which can take single sheet of film through changer from supply magazine to exposure area and then to the receiving cassette.



INSIDE OF AN AOT RAPID FILM CAHNGER

4.12.1 LOADING MAGAZINE: It consists of a large steel container of fixed size to accept 12” x10” film (or 14” x14”). Inside the magazine are arranged a number of strong wore separeators. One film film is loaded into the space between 2 adjacent separators. This magazine will accept upto 30 films loaded in the darkroom. The loading magazine brought from dark room is care fully inserted in to the AOT changer through the window on the left side.



4.12.2 THE EXPOSURE AREA: This is a rectangular area slightly recessed in the top top of the changer. This area is defined by a rectangular grid. Below this is a short short aluminum plate, plate, at the lower surface of which is attached an intensifying screen. screen. Below this aluminums aluminums plats another intensifying screen. It is is fact mounted on a pressure table which is backed with lead and can move up and move down short Fundamentals Of X-Ray Equipments 132


distance when the 2 strong springs holding it up by pressure. The release of this can allow one film to come into come in position between the 2 intensifying screens. screens. A micro-switch I separated as soon as the film comes into position. This micro-switch micro-switch controls the exposure. If a film fails to arrive in position the micro-switch does operate and no exposure takes place. 4.12.3 THE RECIVING CASSETTE: it is a shallow shallow polished metal container capable of holding upto 30 films films of specified specified size. The lid of the cassette is a sliding one which can be pushed down by firm pressure, and is retained in position by a central spring loaded catch. Externally on the changer is a push button which is used to close the receiving cassette at the end o the procedure to procedure to be removed to dark room for the film processing. EXPOSURE AREA

4.12.4 MECHANISM OF FILM TRNSPORT

The automatic drive of AOT changer depends on a motor which is controlled by a thyratron valve. The speed of motor varies with the selection of grid bias voltages. A gear and chain transmission connects the motor assembly to the the screw drive of the changer. A toggle hand – switch initiates the couple pre-selected pre-selected series. This hand- switch has also 2 push button one is for operation and the other is for emergency stop. The exposure occurs within the period of tight compression of the upper and lower pressure plates plates with I.S in in the exposing area. After the exposure the pressure plate releases the film which is now free to be taken by a further set of rollers or wheels which guides into the open lidded receiving cassette. cassette. At the same the, another is on its way to the exposure area. Fundamentals Of X-Ray Equipments 133


This sequence of events is repeated. Each revolution of the screw drive corresponds to the distance between conservative film in the magazine and therefore carries the cassette forward ka sufficient distance to bring the loading film progressively with in the reach of leavers. When the motors stops it should be put into reverse motion by the switch on the control panel meant for this purpose. This sends the empty feed cassette back into its its position and allows it to to be removed subsequently through its door, when the motor will stop automatically. The pile of the films taken out from the receiving cassettes in the same order as they were loaded in the magazine in the dark room.



4.12.5 FILM IDENTIFICATION : There is a device in AOT which automatically marks marks the number on each film. It is situated within the serial serial changer in the side wall where there is also another set for receiving a type written card on which the patients patients name an dID number etc. are written. There Id marks are printed on each film by means of a light flash in addition, the numbers 1 to 30 appears in serial order on the consecutive films or time marking when a switch for this purpose is switched on. 4.12.6 PROGRAMME SELECTION : a) any number of single films upto 30 which can be exposed line any desired time b) a rapid series of upto 30 exposures at present intervals. This selection programme is controlled by means of the toggle control hand-switch. 4.13 CARE AND MAINTENCANCE OF THE X-RAY EQUIPMENTS

X-ray equipments are very expensive to purchase and maintain even their accessories ate very costly. Hence wert have a responsibility for careful treatment of the whole of the x-ray apparatus, especially in an institution, where they are used by different technicians and large number of people are examined. The equipments arte subjected to lot of wear and tear. A radiographer must have a natural respect for the expensive equipment he is handling, lest he will be the person to lay a trail of disaster equivalent to the activity of a purposeful saboteur. Certain simple principles of general mechanical care and maintenance with faithful observation by every technical will help h elp the department. Further a few tests permit p ermit the radiographer radiographe r to know whether the apparatus is working correctly or is faculty. 4.14. GENERAL CARE:

CLEANLINESS: A common cause for trouble is accumulated dust and grit particles in the floor track of the tube stand, these are not readiluy removed by usual procedures of sweeping and mopping of the floor. The dust and grit particle may build up insidiously, to a level at which it becomes difficult difficult to move the the tube column smoothly. smoothly. If any undue force is applied to the tube column moving it against the obstruction, then it will jump off the track. A little periodical (once a week ) attention to the floor track is worth while. It only needs patience and the readiness to kneel with a screw driver of similar tool in the hand and proceed along the track probing and removing the accumulations of flug and similar debris. A stiff brush my be help in removing fine dust . if vacuum cleaner is available, it is best. Liquids are potential hazards for the x-ray department, x-ray tables and film changers do not have water tight covers and liquid used in their vicinity, must be handled carefully with our spillage. A spill can readily provide a short-circuit between 2 electrical point and lead to a major breakdown. In course of barium study, it is not uncommon for Barium drinks and enemas be spilled on the machine. The radiographer must ‘mop up’ at once or atleast Fundamentals Of X-Ray Equipments 135


at the end of the study because if Barium dries up and hardens, its removal becomes rather difficult. During I.V.U I.V.U the doctor giving injection will inadvertently inadvertently spill some of the contrast on the table as he holds syringe vertically and pushes the plunger upwards in order to ensure that no air is contained in the syringe. syringe. This can result in a jut of contrast agent falling any where, even on the x-ray table. So a regular cleaning drill with wet cloth is very important. Otherwise, they will cause artifacts artifacts on the urography films films that follow; A daily damp dusting of each x-ray room and its equipment is a minimum requirement. Whenever necessary, cleaning with Hydrogen peroxide or methyl ate spirits fro removal of bloodstains and other organic sticking materials on the equipment must be done. This helps to keep up the polish on the machine parts. During a fortnightly checkup the loose screws or bolts may be tightened, when the unit unit is in the the switched of condition. Any missing screws of homeless screws are of found to be present must be reported to the chief Radiographer together with any minor faults such as broken meter glasses, reached plastic components or worn out covering etc., The same precautions apply to the mobile or portable equipment. These equipments must be brought back to main x-ray department at least once a month and cleaned properly . if they are parked in wards and operation theaters, these must be well protected from dust by provision of a large polythene or cloth cover. 4.15. MAINTENCE OF LOG BOOK :

This is an important record of a satisfactory maintenance. In such a book, faults must be recorded by the radiographers as they occur and are reported to the Chief Radiographer. This helps the engineer who visits subsequently, to know what are the chronology of events., and to spot the mistake without any waste of time. Finally after the equipment is repaired a brief resume resume of action taken by the engineer in rectifying faults may also be entered. This record has the following advantages: a) The signs and symptoms are brought to the attention of the engineer who has come to come to attend the major failure . b) It provides a firm evidence of particular fault having occurred during the handling by a particular radiographer which may be helpful in subsequent enquiry. c) Like a patient’s case record it provides information about the equipment ‘s history of events to a new or make shift engineer coming in the place of the regular engineer. This is especially so, when the previous engineer has done alterations in the circuits for some reasons. 4.16 PRACTICAL PRECAUTIONS: It is an odd fact that many radiographers have few habits in common. But if these bad habits are repeated often, by minor mistreatment of the equipment, will lead to eventual breakdown implicating not only heavy expenditure but dislocation of work and long wait for the patients. Certain precautionary measure measure are necessary, in dealing with with Brakes of locks, H.T. cables, Meters and controls, tube stand and tracks and other accessory equipments. A few functional tests are also important precautions in maintenance of x -ray equipments Fundamentals Of X-Ray Equipments 136




4.17 BRAKES AND LOCKS:

Some radiographers have a tendency to move the equipment against a firm brake. Eg. X-ray tube raising or lowering or rotating, tube column against rakes etc., Locks of electromagnetic variety can resist such assaults assaults but not a mechanical friction type brake. These mechanical locks must not be over-tightened, but be just turned enough to keep the equipment under control in its desired place. A half turn is sufficient to release it. it. A habit of not loosening the brake perhaps arises out of a compulsion to save time during a busy schedule, but this is to be avoided in the long term interest of the equipment.

4.18 H.T CABLES : o

In some units where the tube can be rotated on its horizontal arm about 360 on the vertical support. The H.T. cables can become wound around the tube column and may result in a) the mobility of the tube become increasingly restricted b) sooner or later the cables will fracture. For this reason, the radiographer must study the position of H.T cable before actually rotating the tube. Secondly the equipment must not be pulled to position by the the cables (eg. Foot switch switch ). We must remember that the points of electrical con nection will not with stand handing strains.

4.19. METERS AND CONTROLS:

Meters so not receive the attention of the radiographers that they deserve. More neglected is mA and mAs meters, this is because of the fact that most of them keep seeing the patient during exposure and not at the meter. A radiographer will discovfer operational during exposure period. This will often prevent totally black radiographs after proce ssing due to lack of exposure.

4.20. TUBE STANDS AND TRACKS:

In moving the tube any distance, it is important to allow it to gallop along and hit the and stops of the track at full speed, otherwise, the tube column will jump off the rails or there is possibility of fracture the filament of the tube insert or even shatter the insert itself. Further more, tube insert can get damaged if the tube is first pushed up with a jerk upward or downwards. So these movements of the tube stands along the tracks and their vertical journey must be handled as gently an possible, after loosening their locks and at the end of the required movement, the locks must be just tightened to keep them in desire position.



4.21. ACCESSORY EQUIPMENTS:

Many things pertaining to x-ray department have to be carried from place to place an dare sometimes dropped carelessly. These are the cones, stationery grids and x-ray cassettes. These are expensive items and proper care must be exercised on them. If these items are found damaged in any way, its condition must reported and registered at once, in the the log book. To continue to use them is such condition will be inviting trouble like jamming of the cassette occurring in the spot film serial changer. No piece of equipment must be subjected to force like fitting a spoiled spoiled cone (whose edge has been bent) to the x-ray tube. A stationary grid grid which is bigger in size that the cassette itself must not be used underneath, a heavy patient as it will become curved and crack c rack eventually. As grids and cons are delicate an d expensive, they must be used with utmost care without without the dropping them. A damaged grid cannot repaired.

4.22. FUNCTIONAL TESTS: The spinning top, the step wedge, the pinhole camera tests: 4.22.1 THE SPINNINGTOP:

This is a simple and useful gadget capable of providing a certain amount of information about the performance of an x-ray unit. It consists of metal disc ( steel or brass ) which is few millimeters thickness and is designed to revolve easily upon a central peg, which has a flatten base. The whole mechanism can be made to stand on a cassette placed place d on the x-ray table and the disc made to to rotate. At a point near the periphery of the disc, disc, is drilled a small hole which is circular. The spin top can be used to test. 1) The timers accuracy 2) The Rectifier valves 3) The filament boost 4) The contactors Regarding the timer the following information can be obtained: A) Whether the time is accurate. B) If inaccurate the extent of the inaccuracy C) Whether the timer is correctly correctly phased with A.C cycle.



4.22.2 THE STEP WEDGE: This is used to confirm that the H.T generator has been correctly calibrated at the time of manufacture and then at installation. This device is small staircase of metal 14 steps each of 3mm thick made of aluminum suitable for use upto 130 KV. The device is kept on the top to xray cassette. The radiograph obtained shows, a stepwise variation of blackening in accordance with the step by step step alteration in thickness of the wedge. Under similar condition of mAs and KV the pattern produced by step wedge must be always the same unless there is fault in calibration of KV or mAs, usually radiographs are taken at 70 KVP with 50 mA, 100mA 25mA and 400 mA with 2 seconds set time standard. Any decomposition of KV can be diagnosed by this



The principle of this test is 2 fold. They are I) the farther away is the object from the pinhole, the smaller will be its image, conversely 2) The ratio object ob ject sizelimage size is equal to the ratio-distance of object from pmhole/distance of Image from pinhole So it is to be inferred that if the object-pin hole distance is kept eqllal to the image pinhole distance then we will have dimensions of the image are the same a those o the object. If an x-ray exposure is made with film on the x-ray table and a lead sheet with a pin hole at its centre, so that the central ray of the x-ray beam passes through it and such that the distance between tube target and lead sheet is equal to that of the lead sheet and the film. We can, record the image of the focal area of the target of the tube on the film which will be a true representation of the focal spots dimensions. Distortion of the target due to pitting of the anode target as a result of over heating will be seen on the pinhole radiograph. A pin hole radiograph is also employed some times to locate the target area accurately in the tube shield i.e., to detect the anode film distance which in turn is very important for such radiographic procedures, as the depth localisation of foreign bodies. t.23. FAILURE OF X-RAY TUBES: A typical sequence of x-ray tube failure be gradual loss of vacuum as a result of over heating of anode and liberation of occluded gasses. Metal from anode is s:Jrayed by electron bombardment round the sides of x-ray tube. When ever the conduction through the tube is altered by factors like these, the mA meter is a good witness. Instability of current will be reflected in a similar fashion in mA and mAs meters, their needles are seen to swing sharply and erratically across their dials. This means that the tube has failed. If a repeat test is to be made with radiographic factors, it must be resisted. Any further, investigations in these lines must be made with fluoroscopic switch, which energises the tube to not more than 2-5 mA. This will save the other components in the circuit, particularly the H. T. rectifiers from further damage.



4.24. FAILURE OF H.T.CABLES: A breakdown in the insulation of the cables will cause H T. current to track to the earth with noisy side effects and abrupt swinging of mA meter needle. When the cable fault is suspected, the anode cable and the cathode cable must be disconnected from their respective receptacles in the tube after switching off the mains supply. Another important point is that after disconnecting each of the cable, its end should be held against some metal part of the x-Tay unit to discharge the residual HT. in it. A spark may often be seen or crackle of electricity heard when this is done. This precaution must be observed when either end of each o f the cable is disconnected for examination. After the cable is disconnected, its tapered ends show the evidence ofHT. Tracking The burned copper pathway. An air gap at the terminal is the most likely cause ofHT. tracking. X-ray ionise air which causes the greatest risk for electrical insulation. If the break in the insulation is elsewhere in the cable, other' than its terminal, a further test may be done. This consists of keeping the cable ends on a wooden chair, mains supply is switched on and the fluoroscopic foot switch is briefly depressed. If there was fault in the cable, any where in its length, the short circuit will lead to erratic sway of needle in mA meter which would not move otherwise, if the cable and tube were normal. A cable may fail, if one of its conductor in it is fractured, in which case, a gentle manipulation of the cable intermittently restores the tube current similar to that of the case of breakage of tube filament from which is has to be differentiated. d ifferentiated. To make this differential diagnosis, the cathode cable is detached from the tube shield, and the x-ray mains and set is switched on. There are 3 pins in the cables termination 1st for small focus filament lead, 2nd for the large focus, the 3rd for connection terminal for both (the pin below the key is for common focus, the one at 3 0' clock position is for fine focus and the one at 9 0' clock position is for large focus) After having known the identity of the pins, the connections are made between the common c ommon focus with the fine focus by b y the help of a well insulated screw driver. If the cable were to be sound, there will be a spark, similarly the conne~tion is made later between the common focus and the large focus and the test for the spark is made. If the spark is not obtained it means that the cable is faulty. If the spark is obtained then the tube filament is faulty. . If the cathode cable is found faulty, then the anode cable can be interchanged in its position. This is because that the anode cable needs only one conductor where as the cathode cable needs 3 conductors. If such interchange is made, this must be recorded in the log book so that the new engineer is informed when he comes for service after this change.


Xray Equipment

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