‐ I. Phased Array: Array: Basic Basic Theory Theory as c r nc pa s Beam
steering
Beam
Focusing
II. Phased Array: The Views Type
of Scans of Scans
Viewing
defects on the S‐Scan
Questions
. Probes Wedges
I. Phased Array: Array: Basic Basic Principals Principals Bell
I. Phased Array: Array: Basic Basic Principals Principals Producing Ultrasound
Pulser
Probe
I. Phased Array: Basic Principals Square Pulse
PW =
500
= z
500
= 100ns
Pulse Height is function of the voltage
100 ns
80 40 V
I. Phased Array: Basic Principals Pulse Length & Height
Small bell (high frequency) = small pulse Big bell (low frequency) = large pulse
u se e g
s re a e
o vo age vo ume o soun :
Small pulse height = weak sound Large pulse height = loud sound
5 MHz 2 MHz = 100 = ns 250 ns
40 V
Dong!
40 V
Ding!
I. Phased Array: Basic Principals Wavelets
Conventional
Phased-Arra
=
element
ultrasound waves originating from the multiple elements
I. Phased Array: Basic Principals Wave Front ‐ result will be a single wave front
,
I. Phased Array: Basic Principals Beamforming, Delay & Sum Delay lines
Digitalising
Sum
+ A‐Scan
Transmission Echo
S‐Scan
I. Phased Array: Basic Principals Snell’s Law
s n α
V
1
α
V 1 = 2,340 m/s = β
Focal point
=
sn
V
2
I. Phased Array: Basic Principals Focal Law: TOF Fermat principle: path between two points.
2,340 m/s
e m i T Focal point
10μs 8μs 6μs 5μs 6μs 7μs 8μs 9μs
I. Phased Array: Basic Principals Focal Law: Delays To calculate the delay for each elements: The longest total sound path of all elements is determined From the value of that sound path the value of a given sound path is 10μs - 8μs = 2μs This
gives the firing delay for each element
0μs 2μs 4μs 5μs 4μs 3μs 2μs 1μs
I. Phased Array: Basic Principals Coverage Conventional UT Raster needed Phased-Array UT No need to raster w o e reg on can e covere
350
n a s ng e pass
700
I. Phased Array: Basic Principals Angular Resolution Angular resolution:
S‐Scan is composed of A‐Scan Angular resolution is the number of A‐Scan per degree
Low angular resolution High angular resolution
I. Phased Array: Basic Principals Digitalization: Resolution
The analog A‐Scan is digitalized with a finite number of points depending on the digitalizing frequency (typically 100MHz) If the number of point is insufficient, the signal will be distorted and could miss vital information. Hence the importance of a high digitalization rate
TOF
I. Phased Array: Basic Principals Digitalization: Resolution: Effect 5MHz Analog Signal
3.125 6.25 12.5 100 50 25 MHz MHz MHz === 918 144 36 72 288 samples samples samples samples
I. Phased Array: Basic Principals Digitalization: Compression
To keep files size manageable, compression is used , This point is then positioned in the middle of the range
I. Phased Array: Basic Principals Digitalization: Compression: Effect
Low number of points High number of points
I. Phased Array: Basic Principals Filters High
Pass: Defines the lowest frequency allowed Low Pass: Defines the highest frequency allowed Band Pass: Centers the filters on the frequency
% ( e d u t i l p m
Frequency (MHz)
I. Phased Array: Basic Principals Filters: Effect
No Filter on a 10 MHz signal Band Pass: For a 10 MHz signal (5.5 MHz to 15 MHz) ) % e d u t i l p m A
Frequency (MHz)
I. Phased Array: Basic Principals Smoothing: Tomoview
Non‐smoothed 10 MHz signal Smoothing is done with a Low pass Filter
) % ( e d u t i l p m A
requency
z
I. Phased Array: Basic Principals Smoothing: Effect 10 MHz Probe
I. Phased Array: Basic Principals PRF: Pulse Repetition Frequency PRF on the Omniscan:1(5)
5 = Number of A‐Scan per second. Depending on the unit one A‐Scan can be composed of 16 to 128 pulses from individual elements 1 = Number of S‐Scan per second
1 second = 5 1 A-Scan S-Scan
Pulsers/Receivers
I. Phased Array: Basic Principals PRF: Example 1
S‐Scan from 35 to 75 degrees with a beam every degree. Thus 40 A‐Scans PRF at 200 Hz (200 A‐Scans / second). Thus 200/40 = 5 S‐Scan / second Encoder is set at one S‐Scan/mm. Max acquisition speed = 5 mm / second 1 second 5 mm
Scan axis
mm
750
350
I. Phased Array: Basic Principals PRF: Example 2
S‐Scan from 35 to 75 degrees with a beam every degree. Thus 40 A‐Scans L‐Scan 14 elements with steps of 1 on a 64‐element probe. Thus 50 A‐Scans PRF at 180 Hz (180 A‐Scans/second). Thus 180/90 = (2 S‐Scan + 2 L‐Scan) / second Encoder is set at one (S‐Scan + L‐Scan)/mm. Max acquisition speed = 2 mm / second 2 seconds Scan axis
mm
4 mm
750
350
I. Phased Array: Basic Principals PRF: Ghost Echo PRF too high for the length of the path OK: The end of the window finishes before the first echo from the next scan arrives PRF too high: The first echo from the next scan is recorded in the previous scan In doubt, lower the PRF. If it is a ghost echo, it should disappear
Ghost Echo
‐ I. Phased Array: Basic Theory Beam steering Beam
Focusing
II. Phased Array: The Views Type
of Scans
Viewing
defects on the S‐Scan
Questions
. Probes Wedges
I. Phased Array: Beam Steering Definition
Is the capability to modify the refracted angle of the beam generated by the array probe
Allows
for multiple angle inspections, using a single probe
Applies
asymmetrical (e.g., linear) focal laws
Can only be performed in steering plane , when using 1D‐arrays
Can generate both L (compression) and SV (shear vertical) waves, usin a sin le robe
I. Phased Array: Beam Steering Principals If a delay is applied between the firing of each element, the resulting wave
I. Phased Array: Beam Steering Limits The limits of beam steering are mainly determined by the size of the elements. The smaller the elements the higher the steering limits. Frequency will also influence the limits. λ v 1 Maximum Steering angle (‐6dB) is given by: sin θ st = 0.5 * = 0.5 * * e f e
16 X 4mm = 64mm aperture 9 degrees
16 X 2mm = 32mm aperture 18 degrees
32 degrees
I. Phased Array: Beam Steering Calculation Maximum
steering angle (at –6 dB), given by:
Sin Ѳst = 0.514 * λ/e Maximum
steering angle (at –20 dB), given by:
st =
.
λ=c/f
= divergence half ‐angle
I. Phased Array: Beam Steering Angle Gain Difference
In PAUT as in conventional UT, there is an optimal angle for each wedge. This optimal angle is determine by the angle of the wedge itself and Snell’s law. The strength of the response obtain from a side drill hole will be maximal at this angle. All other angles below and above will have a lesser response.
⎡υ * sin α ⎤ β = sin −1 ⎢ 1 ⎥ ⎣ υ 2 ⎦
Amplitude vs. Angle
1
α
Ʋ 2
β
) B d ( n ‐5 o i t a i r a v e d u t i l ‐10 p m A
‐15 ‐60
‐50
‐40
‐30
‐20
‐10
0
10
20
Angle in Steel S‐Wave (degree)
30
40
50
60
I. Phased Array: Beam Steering Range: 0 degree wedge
I. Phased Array: Beam Steering Range: 35 degree wedge
‐ I. Phased Array: Basic Theory Beam
steering
Beam Focusing
II. Phased Array: The Views Type
of Scans
Viewing
defects on the S‐Scan
Questions
. Probes Wedges
I. Phased Array: Beam Focusing Definition eam ocus ng
Is the capability to converge the acoustic energy into a small focal spot
Allows
for focusing at several depths, using a single probe
Symmetrical
(e.g., parabolic) focal laws (time delay vs. element position)
Is limited to Phased‐array probe near ‐ field only ,
‐
I. Phased Array: Beam Focusing Principals If the applied delays are calculated so all the singles waves from each elements arrive at the same time on a specific spot, the PA beam will be focused
I. Phased Array: Beam Focusing Near‐Field 1 mm pitch 12 elements Conventional near-field
N =
2
A f
4v
Composite near-field
NC=(1)2*5/(4*5,890) = 0,21 mm Ncom=(1*12)2*5/(4*5,890) = 30,6 mm
I. Phased Array: Beam Focusing Near‐Field: Effective Active Aperture Real
angle and beam dimension Effective angle and beam dimension Effective Aperture is given by the equation: A cos β R eff
cos α I
A
Where αI and βR are:
αI Aeff
βR Focal Point
I. Phased Array: Beam Focusing Near‐Field: Minimum Active Aperture Minimum
active aperture is the minimum active aperture needed to focus at a specific depth along the beam of the maximum refracted angle Minimum Active Aperture is given by the equation: Amin =
⎡ F ⎢ ⎣
2
−
2
⎤
2
*
R
2
f *υ R * cos β
R
⎥ ⎦
A
Where: αI
νI = velocity in first medium (wedge, water) νR = velocity in test piece f = ultrasound frequency F = focal depth for maximum refracted angle =
βR Focal
I. Phased Array: Beam Focusing Near‐Field: Recommended Passive Aperture
The passive aperture is the element length in the non‐active axis. To optimize the beam shape, there is a recommended passive aperture The recommended passive aperture is given by the equation: W = 1.4
Where
λ F
+ F
0.5
:
Fmin = minimal focal depth Fmax = maxima oca ept λ = Wave length
Passive Focalization Active Focalization
I. Phased Array: Beam Focusing Near‐Field Focused Beam: Divergence
0.44λ
=
L
L
Beam
Φ
W
=
= L
(half angle θ, at –6 dB ) W
= a sin
dimension (at depth z)
* . W
2 * 0.44λ z L
0.44λ W
I. Phased Array: Beam Focusing Type of Focus
True Depth
Half Path
Projection
Focal Plane (3- D)
I. Phased Array: Beam Focusing Type of Focus: True Depth
32 elements of 0.6mm
I. Phased Array: Beam Focusing Type of Focus: Half Path
32 elements of 0.6mm
I. Phased Array: Beam Focusing Type of Focus: Projection
32 elements of 0.6mm
I. Phased Array: Beam Focusing Number of Elements: Focus 50mm
16 elements of 0.6mm . 64 elements of 0.6mm
I. Phased Array: Beam Focusing Number of Elements: Focus 100 mm
16 elements of 0.6mm . 64 elements of 0.6mm
I. Phased Array: Beam Focusing Focal Zone Vs Signal amplitude
On a 50mm thick part, the focal distance is set at 25 mm , change along the beam It will be lower in the near and far fields And higher in the focal zone
I. Phased Array: Array: Beam Focusing Dynamic Depth Focusing (DDF) ne oca aw s use Several focal
n
laws are used in RX
The beam spot produced by the DDF is equal or smaller than the one produced by standard phased‐array.
S/N ratio is equivalent or higher than the one obtained with standard Phased Array
The use of of DDF DDF creates very small beam spread
I. Phased Array: Array: Beam Focusing Dynamic Depth Focusing
=
Rx 3 Rx 2 Rx 1
I. Phased Array: Array: Beam Focusing Dynamic Depth Focusing
=
Rx 3 Rx 2 Rx 1
I. Phased Array: Beam Focusing Dynamic Depth Focusing single pulse. The beam is refocused electronically on its return.
Focus Depth (Pulser)
I. Phased Array: Beam Focusing Dynamic Depth Focusing
Standard focusing
DDF
I. Phased Array: Basic Principals Definitions Focal Law:
A file which defines the elements to be fired time dela s volta es for both the transmitter and receiver functions.
Beam steering: Capacity
to electronically steer the phased‐array ultrasound beam
Beam Focusing: Capacity
to electronically change the focal point of a phased‐array beam
Delay and Sum beamforming:
Creating a composite A‐Scan from several individual A‐Scans (one for each individual elements)
S‐Scan: Sectorial
scan: Represents a slice of the part between set angles
Linear Scanning: Electronic
scanning using a group of elements from a probe with a higher number o e emen s en e group s
‐ I. Phased Array: Basic Theory Beam
steering
Beam
Focusing
II. Phased Array: The Views Type
of Scans
Viewing
defects on the S‐Scan
Questions
. Probes Wedges
I. Phased Array: Questions Question 1 Q: If A is for a 5 MHz at 80V, what is B:
A
B
a)
A spike pulse 200ns, 80V
b)
A square pulse 200ns, 80V
c)
A square pulse 200ns, 40V
I. Phased Array: Questions Answer Q1 A: C) Square pulse 200ns 40V
40 V
The frequency of the probe defines the length of the pulse and the voltage
I. Phased Array: Questions Question 2 Q: Identify the following:
a)
UT probe
b)
PAUT probe
c)
TOFD probe
I. Phased Array: Questions Answer Q2 A: b) PAUT probe
The active surface of the probe is divided in several elements (6)
I. Phased Array: Questions Question 3 Q: What differentiate a PAUT A‐Scan from a conventional UT A‐Scan ?
I. Phased Array: Questions Answer Q3 :
‐ can s a compos e ‐ can. e from each elements of the probe. e ay lines
Digitalising
Sum
+
s a summa on o a
Composite A-Scan
‐ can
I. Phased Array: Questions Question 4 Q: When creating a UT beam at a certain angle, are all the soundpath lengths
equal?
a) b) c)
Generally, yes Depends on the focus Depends on the voltage ,
I. Phased Array: Questions Answer Q4 : n
eac wave produced by the different elements have a different soun pat engt . owever there is one exception, when the beam is at zero degree and focused at infinite.
2,340 m/s
Focal point
I. Phased Array: Questions Question 5 Q: What would be the length of the near field, in steel L‐wave (5.9 mm/μs), for a 5MHz Phased‐array probe?
7 mm
b)
0,21 mm
c)
10,40 mm mm
N=D2f/(4c)
I. Phased Array: Questions Answer Q5 A: C) 10,40 mm. A phased‐array probe can only focus within its near field.
7 mm
N=D2f/(4c)
N=72*5/(4*5,9) = 10,40 mm
I. Phased Array: Questions Question 6 Q: Which of the following set of delays will create a focused phased‐array beam?
a)
b)
c)
d) None of the above
I. Phased Array: Questions Answer Q6 Q: d) None of the Above
A focused beam is produced by calculating the firing delay for each individual elements so all of the individual waves will arrive at a focal point at the same .
I. Phased Array: Questions Question 7 Q: The size of individual elements is more important for what?
a)
Focusing
b)
Steering
c)
Dynamic focusing
I. Phased Array: Questions Answer Q7 A: It is important for steering. In general the smaller the elements, the better
the steering.
I. Phased Array: Questions Question 8 Q: Identify the following:
Rx 1 2 3
a)
Sectorial Scan
b)
Linear / Electronic Scan
c)
DDF
