(eBook) MRI for Dummies [Risonanza Magnetic A]

Other Resources These slides were condensed condensed from several excellent excellent online sources. I have tried to give credit where appropriate. If you would like a more thorough introductory review of MR physics, I suggest the following: Robert Cox’s slideshow, (f)MRI Physics with Hardly Any Math, Math , and his book chapters online. http://afni.nimh.nih.gov/afni/edu/ See “Background Information on MRI” section Mark Cohen’s intro Basic MR Physics slides http://porkpie.loni.ucla.edu/BMD_HTML/SharedCode/MiscShared.html Douglas Noll’s Primer on MRI and Functional MRI http://www.bme.umich.edu/~dnoll/primer2.pdf

For a more advanced tutorial, see: Joseph Hornak’s Web Tutorial, The Basics of MRI http://www.cis.rit.edu/htbooks/mri/mri-main.htm

Recipe for MRI 1) Put subject in big magnetic field (leave him there) 2) Transmit radio waves into subject

[about 3 ms]

3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject – Manipulat Manipulate e re-transm re-transmissio ission n with magne magnetic tic fields fields during during this this readout interval [10-100 ms: MRI is not a snapshot]

5) Store measured radio wave data vs. time – Now go back back to 2) 2) to get get some some mor more e data data

6) Process raw data to reconstruct images 7) Allow subject to leave scanner (this is optional)

Source: Robert Cox’s web slides

History of NMR NMR = nuclear magnetic resonance Felix Block and Edward Purcell 1946: atomic nuclei absorb and reemit radio frequency energy 1952: Nobel prize in physics nuclear : properties of nuclei of atoms magnetic:: magnetic field required magnetic resonance:: interaction between resonance magnetic field and radio frequency Bloch

Purcell

NMR → MRI: Why the name change?

most likely explanation: nuclear has bad connotations

less likely but more amusing explanation: subjects got nervous when fast-talking doctors suggested an NMR

History of fMRI MRI -1971: MRI Tumor detection (Damadian) ( Damadian) -1973: Lauterbur suggests NMR could be used to form images -1977: clinical MRI scanner patented -1977: Mansfield proposes echo-planar imaging (EPI) to acquire images faster fMRI -1990: Ogawa observes BOLD effect with T2* blood vessels became more visible as blood oxygen decreased -1991: Belliveau observes first functional images using a contrast agent -1992: Ogawa et al. and Kwong K wong et al. publish first functional images using BOLD signal

Ogawa

Necessary Equipment 4T magnet

RF Coil gradient coil (inside)

Magnet

Gradient Coil

RF Coil

Source: Joe Gati, photos

The Big Magnet Very strong 1 Tesla (T) = 10,000 Gauss Earth’s magnetic field = 0.5 Gauss 4 Tesla = 4 x 10,000 ÷ 0.5 = 80,000X Earth’s magnetic field Continuously on Main field = B0 Robarts Research Institute 4T

x 80,000 =

B0

Source: www.spacedaily.com

Magnet Safety The whopping strength of the magnet makes safety essential essential.. Things Thing s fly – Even big big things! things!

Source: www.howstuffworks.com

Source: http://www.simplyphysics.com/ flying_objects.html

Screen subjects carefully Make sure you and all your students & staff are aware of hazzards Develop stratetgies for screening yourself every time you enter the magnet

Do the metal macarena!

Subject Safety Anyone going near the magnet – subjects, staff staff and visitors visitors – must be thoroughly screened: Subjects must have no metal in their bodies: bodies: • pac pacema emaker ker • aneur aneurysm ysm clips • metal implants implants (e.g., cochlear implants) implants) • inter interuteri uterine ne devices (IUDs) (IUDs) • some dental work (fillings (fillings okay) okay) This subject was wearing a hair band with a ~2 mm Subjects must remove metal from their bodies copper clamp. clamp. Left: with hair band. Right: without. • jewe jewellery llery,, watc watch, h, piercings piercings Source: Jorge Jovicich • coi coins, ns, etc. etc. • wa wall llet et • any metal that may distort the the field (e.g., underwire underwire bra)

Subjects must be given ear plugs (acoustic noise can reach 120 dB)

Protons Can measure nuclei with odd number of neutrons 1H, 13C, 19F, 23Na, 31P 1H

(proton) abundant: high concentration in human body high sensitivity: yields large signals

Outside magnetic field

Protons align with field • randomly oriented

Inside magnetic field

M

• spins tend to align parallel or anti-parallel to B0 • net magnetizati magnetization on (M) along B0 • spins precess precess with random random phase • no net magnetization magnetization in transverse transverse plane • only 0.0003% of protons/T protons/T align with with field longitudinal axis

Longitudinal magnetization

M=0

Source: Mark Cohen’s web slides Source: Robert Cox’s web slides

transverse plane

Larmor Frequency Larmorr equa Larmo equation tion f = γB0 γ = 42.58 MHz/T At 1.5T, f = 63.76 MHz At 4T, f = 170.3 MHz

170.3

Resonance Frequency for 1H 63.8

1.5

4.0

Field Strength (Tesla)

RF Excitation Excite Radio Frequency (RF) field • transmission coil: coil: apply magnetic field along B1 (perpendicular to B0) for ~3 ms • oscilla oscillating ting field at Larmor Larmor freque frequency ncy • frequen frequencies cies in range of radio transmissions transmissions • B1 is small: ~1/10,000 T • tips M to to transverse transverse plane plane – spirals down • analogies: guitar string (Noll), swing (Cox) • final angle angle between between B0 and B1 is the flip angle

Transverse magnetization

B0

B1


Cox’s Swing Analogy


Relaxation and Receiving Receive Radio Frequency Field • receiving coil: coil: measure net magnetization (M) • readout interval (~10-100 ms) • relaxation relaxation:: after RF field turned on and off, magnetization returns to normal longitudinal magnetiz magnetization ation↑ → T1 signal recovers transverse magnetization ↓ → T2 signal decays


T1 and TR T1 = recovery of longitudinal (B 0) magnetization • use us ed in in an anat ato omi mica call im ima age ges s • ~500 ~5 00-1 -100 000 0 mse msec c (lo (long nger er wi with th bi bigg gger er B0 B0)) TR (repetition time) = time to wait after excitation before sampling T1

Source: Mark Cohen’s web slides

add a gradient to the main magnetic field

Spatial Coding:Gradien Coding:Gradients ts How can we encode spatial position?

excite only frequencies corresponding to slice plane

• Example: axial slice slice

Use other tricks to get other two dimensions • left-righ left-right: t: frequency frequency encode • top-bot top-bottom: tom: phase phase encode

q e r F

Field Strength (T) ~ z position

Gradient switching – that’s what makes all the beeping & buzzing noises during imaging!

Gradient coil

Precession In and Out of Phase

• protons precess at slightly different frequencies because of (1) random fluctuations in the local field at the molecular level that affect both T2 and T2*; (2) larger scale variations in the magnetic field (such as the presence of deoxyhemoglobin!) that affect T2* only. • over time, the frequency differences lead to different phases between the molecules (think of a bunch of clocks running at different rates – at first they are synchronized, but over over time, they get more and more out of sync until they are random) • as the protons get out of phase, the transverse magnetization decays • this decay occurs at different different rates in different tissues tissues


T2 and TE T2 = decay of transverse magnetization TE (time to echo) = time to wait to measure T2 or T2* (after refocussing with spin echo or gradient echo)


Echos pulse sequence: sequence : series of excitations, gradient triggers and readouts Gradient echo Echos – refocu refocussing ssing of signal signal pulse sequence Spin echo: echo: use a 180 degree pulse to “mirror image” the spins in the transverse plane when “fast” regions get ahead in phase, make them go to the back and catch up

-measure T2 -ideally TE = average T2 Gradient echo: echo: flip the gradient from negative to positive t = TE/2

make “fast” regions become “slow” and vice-versa

A gradient reversal (shown) or 180 pulse (not shown) at this point will lead to a recovery of transverse magnetization

-measure T2* -ideally TE ~ average T2* TE = time to wait to measure refocussed spins


T1 vs. T2


K-Space

Source: Traveler’s Guide to K-space (C.A. Mistretta)

A Walk Through K-space single shot

two shot

K-space can be sampled in many “shots” (or even in a spiral)

Note: The above is k-space, not slices

2 shot or 4 shot • less time between samples samples of slices slices • allow allows s temporal interpolation interpolation

vs.

both halves of k-space in 1 sec

1st half of k-space in 0.5 sec

2nd half of k-space 1st half of k-space in 0.5 sec in 0.5 sec

1stt vol 1s volum ume e in in 1 se sec c

inte in terp rpol olat ated ed image

2nd half of k-space in 0.5 sec

2nd volume in 1 sec

T2* T2* relaxation • deph dephasin asing g of transverse transverse magnetiz magnetization ation due due to both: - microscopic molecular interactions (T 2) - spatial variations variations of the external main main field ∆B (tissue/air, tissue/bone interfaces) • expo exponenti nential al decay decay (T2* ≈ 30 - 100 ms, shorte shorterr for higher higher Bo) Mxy Mo sin T2 T2*

time Source: Jorge Jovicich

Susceptibility Adding a nonuniform object (like a person) to B0 will make the total magnetic field nonuniform This is due to susceptibility susceptibility:: generation of extra magnetic fields in materials that are immersed in an external field For large scale (10+ cm) inhomogeneities, inhomogeneities, scanner-supplied scanner-supplied nonuniform magnetic fields can be adjusted to “even out” the ripples in B — this is called shimming

sinuses ear canals

Susceptibility Artifact -occurs near junctions between air and tissue • sinuses sinuses,, ear canals -spins become dephased so quickly (quick T2*), no signal can be measured

Susceptibility variations can also be seen around blood vessels where deoxyhemoglobin affects T2* in nearby tissue Source: Robert Cox’s web slides

Hemoglobin

Hemoglogin Hemog login (Hgb): - four globin chains - each globin chain contains contains a heme group group - at center of each each heme group is an iron atom (Fe) - each heme group group can attach an oxygen oxygen atom (O2) - oxy-Hgb (four O2) is diamagnetic → no ∆B effects - deox deoxy-H y-Hgb gb is paramagnetic → if [deoxy-Hgb] ↓ → local ∆B ↓

Source: http://wsrv.clas.virginia.edu/~rjh9u/hemoglob.html , Jorge Jovicich

BOLD signal Blood Oxygen Level Dependent signal

↑neural activity  ↑ blood flow  ↑ oxyhemoglobin  ↑ T2*  ↑ MR signal Mxy Signal

Mo sin

T2* task T2* control

Stask Scontrol

S

TEoptimum

Source: fMRIB Brief Introduction to fMRI

time

Source: Jorge Jovicich

BOLD signal

Source: Doug Noll’s primer

First Functional Images

Source: Kwong et al., 1992

Hemodynamic Response Function

% signal change

time to rise

= (point – baseli baseline)/ba ne)/baseline seline usually 0.5-3%

initial dip

signal begins to rise soon after stimulus begins

time to peak signal peaks 4-6 sec after stimulus begins

-more focal and potentially a better measure post stimulus undershoot -somewhat elusive so far, not signal suppressed after stimulation ends everyone can find it

Review Magnetic field

Tissue protons align with magnetic field (equilibrium state) RF pulses

Relaxation processes

Protons absorb Spatial encoding RF energy using magnetic (excited state) field gradients Relaxation processes

Protons emit RF energy (return to equilibrium state) NMR signal detection Repeat RAW DATA MATRIX

Fourier transform

IMAGE

Source: Jorge Jovicich

(eBook) MRI for Dummies [Risonanza Magnetic A]

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