(eBook) MRI for Dummies
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Transcript of (eBook) MRI for Dummies
1
Section 2Basic fMRI Physics
Other ResourcesThese slides were condensed from several 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, 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 slideshttp://porkpie.loni.ucla.edu/BMD_HTML/SharedCode/MiscShared.html
Douglas Noll’s Primer on MRI and Functional MRIhttp://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
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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 transmitter4) Receive radio waves re-transmitted by subject
– Manipulate re-transmission with magnetic fields during this readoutinterval [10-100 ms: MRI is not a snapshot]
5) Store measured radio wave data vs. time– Now go back to 2) to get some more data
6) Process raw data to reconstruct images7) Allow subject to leave scanner (this is optional)
Source: Robert Cox’s web slides
History of NMRNMR = nuclear magnetic resonance
Felix Block and Edward Purcell1946: atomic nuclei absorb and re-emit radio frequency energy1952: Nobel prize in physics
nuclear: properties of nuclei of atomsmagnetic: magnetic field requiredresonance: interaction between magnetic field and radio frequency
Bloch PurcellNMR → 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
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History of fMRIMRI-1971: MRI Tumor detection (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 et al. publish first functional images using BOLD signal
Ogawa
Necessary Equipment
Magnet Gradient Coil RF Coil
Source: Joe Gati, photos
RF Coil
4T magnet
gradient coil(inside)
4
x 80,000 =
4 Tesla = 4 x 10,000 ÷ 0.5 = 80,000X Earth’s magnetic field
Robarts Research Institute 4T
The Big Magnet
Very strong
Continuously on
Source: www.spacedaily.com
1 Tesla (T) = 10,000 Gauss
Earth’s magnetic field = 0.5 Gauss
Main field = B0
B0
Magnet SafetyThe whopping strength of the magnet makes safety essential.Things fly – Even big things!
Screen subjects carefullyMake sure you and all your students & staff are aware of hazzardsDevelop stratetgies for screening yourself every time you enter the magnet
Do the metal macarena!
Source: www.howstuffworks.com Source: http://www.simplyphysics.com/flying_objects.html
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Subject SafetyAnyone going near the magnet – subjects, staff and visitors – must be thoroughly screened:
Subjects must have no metal in their bodies:• pacemaker• aneurysm clips• metal implants (e.g., cochlear implants)• interuterine devices (IUDs)• some dental work (fillings okay)
Subjects must remove metal from their bodies• jewellery, watch, piercings• coins, etc.• wallet• any metal that may distort the field (e.g., underwire bra)
Subjects must be given ear plugs (acoustic noise can reach 120 dB)
This subject was wearing a hair band with a ~2 mm copper clamp. Left: with hair band. Right: without.
Source: Jorge Jovicich
Protons
Can measure nuclei with odd number of neutrons1H, 13C, 19F, 23Na, 31P
1H (proton)abundant: high concentration in human bodyhigh sensitivity: yields large signals
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Protons align with fieldOutside magnetic field
Inside magnetic field
• randomly oriented
• spins tend to align parallel or anti-parallel to B0• net magnetization (M) along B0• spins precess with random phase• no net magnetization in transverse plane• only 0.0003% of protons/T align with field
Source: Mark Cohen’s web slides
M
M = 0 Source: Robert Cox’s web slides
longitudinalaxis
transverseplane
Longitudinalmagnetization
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Larmor Frequency
Larmor equationf = γB0γ = 42.58 MHz/T
At 1.5T, f = 63.76 MHzAt 4T, f = 170.3 MHz
Field Strength (Tesla)
ResonanceFrequency for 1H
170.3
63.8
1.5 4.0
RF Excitation
Excite Radio Frequency (RF) field• transmission coil: apply magnetic field along B1(perpendicular to B0) for ~3 ms• oscillating field at Larmor frequency• frequencies in range of radio transmissions• B1 is small: ~1/10,000 T• tips M to transverse plane – spirals down• analogies: guitar string (Noll), swing (Cox)• final angle between B0 and B1 is the flip angle
B1
B0
Source: Robert Cox’s web slides
Transversemagnetization
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Cox’s Swing Analogy
Source: Robert Cox’s web slides
Relaxation and Receiving
Receive Radio Frequency Field• receiving coil: measure net magnetization (M)• readout interval (~10-100 ms)• relaxation: after RF field turned on and off, magnetization returns to normal
longitudinal magnetization↑ → T1 signal recoverstransverse magnetization↓ → T2 signal decays
Source: Robert Cox’s web slides
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T1 and TR
Source: Mark Cohen’s web slides
T1 = recovery of longitudinal (B0) magnetization• used in anatomical images• ~500-1000 msec (longer with bigger B0)
TR (repetition time) = time to wait after excitation before sampling T1
Spatial Coding:GradientsHow can we encode spatial position?
• Example: axial slice
Use other tricks to get other two dimensions• left-right: frequency encode
• top-bottom: phase encode
excite only frequencies
corresponding to slice plane
Field Strength (T) ~ z position
Freq
Gradient coil
add a gradient to the main magnetic
field
Gradient switching – that’s what makes all the beeping & buzzing noises during imaging!
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Precession In and Out of Phase
Source: Mark Cohen’s web slides
• 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 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 rates in different tissues
T2 and TE
Source: Mark Cohen’s web slides
T2 = decay of transverse magnetizationTE (time to echo) = time to wait to measure T2 or T2* (after refocussing with spin echo or gradient echo)
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Echos
Source: Mark Cohen’s web slides
Echos – refocussing of signal
Spin 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:
flip the gradient from negative to positive
make “fast” regions become “slow” and vice-versa
-measure T2*
-ideally TE ~ average T2*
pulse sequence: series of excitations, gradient triggers and readouts
Gradient echopulse sequence
t = TE/2
A gradient reversal (shown) or 180 pulse (not shown) at this point will lead to a recovery of transverse magnetization
TE = time to wait to measure refocussed spins
T1 vs. T2
Source: Mark Cohen’s web slides
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K-Space
Source: Traveler’s Guide to K-space (C.A. Mistretta)
A Walk Through K-spaceK-space can be sampled in many “shots”(or even in a spiral)
2 shot or 4 shot• less time between samples of slices• allows temporal interpolation
both halves of k-space in 1 sec
1st half of k-spacein 0.5 sec
2nd half of k-spacein 0.5 sec
vs.
single shot two shot
1st volume in 1 sec interpolatedimage
Note: The above is k-space, not slices
1st half of k-spacein 0.5 sec
2nd half of k-spacein 0.5 sec
2nd volume in 1 sec
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T2*
Source: Jorge Jovicich
time
Mxy
Mo sinθT2
T2*
T2* relaxation
• dephasing of transverse magnetization due to both:
- microscopic molecular interactions (T2)
- spatial variations of the external main field ∆B
(tissue/air, tissue/bone interfaces)
• exponential decay (T2* ≈ 30 - 100 ms, shorter for higher Bo)
Susceptibility
Source: Robert Cox’s web slides
Adding a nonuniform object (like a person) to B0 will make the total magnetic field nonuniform
This is due to susceptibility: generation of extra magnetic fields in materials that are immersed in an external field
For large scale (10+ cm) inhomogeneities, scanner-supplied nonuniform magnetic fields can be adjusted to “even out” the ripples in B — this is called shimming
Susceptibility Artifact-occurs near junctions between air and tissue
• sinuses, ear canals-spins become dephased so quickly (quick T2*), no signal can be measured
sinuses
earcanals
Susceptibility variations can also be seen around blood vessels where deoxyhemoglobin affects T2* in nearby tissue
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Hemoglobin
Source: http://wsrv.clas.virginia.edu/~rjh9u/hemoglob.html, Jorge Jovicich
Hemoglogin (Hgb):- four globin chains- each globin chain contains a heme group- at center of each heme group is an iron atom (Fe)- each heme group can attach an oxygen atom (O2)- oxy-Hgb (four O2) is diamagnetic → no ∆B effects- deoxy-Hgb is paramagnetic → if [deoxy-Hgb] ↓ → local ∆B ↓
BOLD signal
Source: fMRIB Brief Introduction to fMRI
↑neural activity ↑ blood flow ↑ oxyhemoglobin ↑ T2* ↑ MR signal
Blood Oxygen Level Dependent signal
time
MxySignal
Mo sinθ T2* task
T2* control
TEoptimum
StaskScontrol
∆S
Source: Jorge Jovicich
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BOLD signal
Source: Doug Noll’s primer
First Functional Images
Source: Kwong et al., 1992
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Hemodynamic Response Function
% signal change= (point – baseline)/baselineusually 0.5-3%
initial dip-more focal and potentially a better measure-somewhat elusive so far, not everyone can find it
time to risesignal begins to rise soon after stimulus begins
time to peaksignal peaks 4-6 sec after stimulus begins
post stimulus undershootsignal suppressed after stimulation ends
Review
Tissue protons align with magnetic field(equilibrium state)
RF pulses
Protons absorbRF energy
(excited state)
Relaxation processes
Protons emit RF energy(return to equilibrium state)
Spatial encodingusing magneticfield gradients
Relaxation processes
NMR signaldetection
Repeat
RAW DATA MATRIX
Fourier transform
IMAGE
Magnetic field
Source: Jorge Jovicich