Lecture01 CourseIntro MRIPhysicsece-research.unm.edu/vcalhoun/courses/fMRI_Spring...1 Sprint 2009...
Transcript of Lecture01 CourseIntro MRIPhysicsece-research.unm.edu/vcalhoun/courses/fMRI_Spring...1 Sprint 2009...
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Sprint 2009 fMRI Analysis Course Sprint 2009 fMRI Analysis Course 1
Analysis Methods in Analysis Methods in Functional Magnetic Resonance ImagingFunctional Magnetic Resonance Imaging
ECE 595ECE 595--0808
•• Review Syllabus (check Feb 3Review Syllabus (check Feb 3rdrd class)class)
•• LectureLecture
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OutlineOutline•• MR Basic PrinciplesMR Basic Principles
•• HardwareHardware
•• SpinSpin
•• SequencesSequences
•• Basics of BOLD fMRIBasics of BOLD fMRI•• Signal mechanismSignal mechanism
•• Sequences usedSequences used
•• ArtifactsArtifacts
•• A few tradeA few trade--offsoffs
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Puzzle PiecesPuzzle Pieces
Helmholtz
Golay
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The MagnetThe Magnet
•• Goal: align the protonsGoal: align the protons•• Coils Coils
•• Super conductance: Super conductance: HeliumHelium
•• 1.5T, 1.5T, 3T3T, 7T , 7T (Earth magnetic field = (Earth magnetic field = 0.0005T) 0.0005T)
•• Side Effects (FDA : <8T, Side Effects (FDA : <8T, neonates <4T )neonates <4T )•• NauseaNausea•• VertigoVertigo•• TinglingTingling•• HeadacheHeadache•• Pain in tooth fillingsPain in tooth fillings
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Not harmful?Not harmful?
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The Gradient CoilsThe Gradient Coils•• Goal: Goal:
•• Slice selectionSlice selection
•• Frequency encodingFrequency encoding
•• Phase encodingPhase encoding
Side EffectsSide Effects•• Induced currents (dynamo; small) Induced currents (dynamo; small)
•• Nerve stimulation Nerve stimulation
•• PhosphenesPhosphenes
•• Acoustic NoiseAcoustic Noise
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The RF CoilThe RF Coil•• Goal: Goal:
•• Turn longitudinal Turn longitudinal magnetization into magnetization into transverse magnetizationtransverse magnetization
•• Measure the signal Measure the signal generated by the precessing generated by the precessing spins. spins.
•• Side Effects Side Effects •• Induced currents: Specific Induced currents: Specific
Absorption Rate (SAR) Absorption Rate (SAR) limits limits
•• Heating: avoid loops.Heating: avoid loops.
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CoilsCoils
Head coil•homogenous signal•moderate SNR
Surface coil•highest signal at hotspot•high SNR at hotspot
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Basic TheoryBasic Theory
•• Protons have a property called spinProtons have a property called spin
•• LarmorLarmor Equation: Equation: ωω = = γγBB00
•• ωω = = LarmorLarmor frequencyfrequency
•• γγ = = gyromagneticgyromagnetic ratioratio•• 42MHz/T for protons (42MHz/T for protons (11H) H)
•• 11MHz/T for 11MHz/T for 1313CC
•• 176GHz/T for electrons (e176GHz/T for electrons (e--))
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Basic TheoryBasic Theory
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Basic TheoryBasic Theory
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Basic TheoryBasic Theory
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Basic TheoryBasic Theory
180°90°
z
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Basic TheoryBasic Theory
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Mo
Mz
t
63%
T1
Longitudinal Relaxation Time T1
Longitudinal Relaxation = Energy transfer between excited spins andTissue (Spin-Lattice-Relaxation)
Reestablishing of longitudinal magnetization with time constant T1
1-e-t/T1
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Mxy
37%
T2
t
Transverse Relaxation Time T2
Transverse Relaxation = Decay of magnetization by interaction between nuclei (Spin-Spin-Relaxation)
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Mz S
Tissue 1
Tissue 2
TRShort TE Medium TE Long TE
Longitudinal Relaxation Transverse Relaxation
Tissue 2
Tissue 1
Relaxation Times are Tissue Specific
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30002000100000.0
0.2
0.4
0.6
0.8
1.0
TR (msec)
Sig
nal
gray matterT1 = 1000
CSFT1 = 3000
white matterT1 = 600
T1T1--relaxation timerelaxation time
•• Depends on tissue typeDepends on tissue type•• White matter: 70 msWhite matter: 70 ms•• Gray matter: Gray matter: 90 ms90 ms•• CSF: CSF: 400 ms400 ms
•• T2 << T1T2 << T1
•• Measure the signal Measure the signal 200 ms after the RF pulse. 200 ms after the RF pulse.
•• White matter: eWhite matter: e--200/70 200/70 = 5% = 5%
•• Gray matter: eGray matter: e--200/90 200/90 = 10%= 10%
•• CSF:CSF: ee--200/400 200/400 = 60= 60
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TR
Long
Short
Short Long
TE
ProtonDensity
T1 poor!
T2
Image contrastImage contrastNMR Signal
B0
z’ z’ z’
X’ X’ X’
y’ y’ y’
S(t) S(t) S(t)
M0
S(t)
t
S0≈M0 T2*
0
T0=1/f0
Spin refocusing
z’
X’
y’ y’
z’
X’
y’y’
B0
z’
X’
z’
X’
12
3
45
1
23
45
z’
X’
y’1
23
45
90°x 180°y
Hahn echo
CPMG (Carr-Purcell-Meiboom-Gill) modification : multiple
t = TEt = TE/2t = 0
Spin echo
90° 180°
t = TE/2
t = TE
2
Spin echo
FID Spin echo
readout
Signal
Gradient
TransversalMomentsPhase
+
-
0t
1
2
3
4
5
1
4
5
2
3
T2
T2*
90° 180°
Gradient echo
readoutGradient
FID gradientecho
Signal
TransversalMomentsPhase
t
T2*
5
Longitudinal Relaxation
B0
t=t0 t=t1Mz=0
t=t2Mz=a
t=t3Mz=b
t=Mz=1
….
t
Mz(t)
t0 t1 t2 t3
90°M0
Making an image1. Slice selection with a gradient field
B0
0
a) Set a z-gradient b) Choose the frequency of the RF pulsec) Switch off the z-gradient
Resonance at ω = γ(B0+ 1)
B0+1
B0+2
z
Making an image2. Frequency encoding with a gradient field
0x
Faster precession: fast changing signal
Slower precession: slow changing signal
Bx
a) When measuring the signal, set a gradientb) Measure only fast signals -> back of headc) Measure only slow signals -> front of head
Making an image3. Phase encoding with a gradient field
0y
By
a) After the RF pulse, set a gradient for a brief timeb) Measure the signal
The phase of the signal depends on the y-position : sin(…+y)c) Repeat, with ever stronger gradient
The signal : sin(…+2y), sin(…+3y), sin(…+4y)d) Signals that change rapidly with the repeat number have large ye) Signals that change slowly with the repeat number have small y
Phase advance
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1 2 N
ky
kx
Raw Data Matrix (k-Space)
Raw data matrix or k-space is filled line by line by variation of the Phase Encoding Gradient
Line Information =Frequencies of the Readout Gradient
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Fourier Transformation
K_x
K_y
x
y
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EPI imaging and kEPI imaging and k--spacespacex = frequency and y = phase or angle
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Frequency and phase encoding merely Frequency and phase encoding merely plots a trajectory across kplots a trajectory across k--space.space.
frequ. encode
phase encode
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EPI
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EPI and SpiralsEPI and Spirals
kx
ky
Gx
Gy
kx
ky
Gx
Gy
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EPI Spirals
Susceptibility: distortion, blurring,dephasing dephasing
Eddy currents: ghosts blurring
k = 0 is sampled: 1/2 through 1st
Corners of kspace: yes no
Gradient demands: very high pretty high
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OutlineOutline•• MR Basic PrinciplesMR Basic Principles
•• HardwareHardware
•• SpinSpin
•• SequencesSequences
•• Basics of BOLD fMRIBasics of BOLD fMRI•• Signal mechanismSignal mechanism
•• Sequences usedSequences used
•• ArtifactsArtifacts
•• A few tradeA few trade--offsoffs
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Basics of BOLD fMRIBasics of BOLD fMRI
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The MR roomThe MR room
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Scanner InternalsScanner Internals
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Macroscopic: Brain SystemsMacroscopic: Brain Systems
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Microscopic: Neuronal FunctionMicroscopic: Neuronal Function
Action Potentials & Neurotransmitter TraffickingAction Potentials & Neurotransmitter Trafficking
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Hemodynamic Measure of Brain Function (1881)Hemodynamic Measure of Brain Function (1881)
ArmArm
BrainBrain
Angelo Angelo MossoMosso
Pressure TracesPressure Traces ““BertinoBertino””
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Artery Vein
Arterioles Venules
1 - 3 cm
Capillary Bed
Neurons
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Blood Oxygen Level Dependent (BOLD)Blood Oxygen Level Dependent (BOLD)
•• Neural activity Neural activity increasesincreases•• Blood flow Blood flow increasesincreases ((““reactive hyperemiareactive hyperemia””))•• DeoxyhemoglobinDeoxyhemoglobin concentration concentration decreasesdecreases•• Magnetic field homogeneity Magnetic field homogeneity increasesincreases•• Gradient echo EPI signal Gradient echo EPI signal increasesincreases
venulesvenulesarteriolesarterioles
BaselineBaseline
capillarycapillarybedbed
time
MxySignal
Mo
sin T2* taskT2* control
TEoptimum
Stask
ScontrolS
time
MxySignal
Mo
sin T2* taskT2* control
TEoptimum
Stask
ScontrolS
““ActivatedActivated””
venulesvenulesarteriolesarterioles
NeuronalNeuronalFiringFiring
capillarycapillarybedbed
HbOHbO22
DeoxyDeoxy--HbHb
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Hemodynamic Response PropertiesHemodynamic Response Properties•• Magnitude of signal changes is quite small Magnitude of signal changes is quite small
•• 0.5 to 3% at 1.5 T (or smaller)0.5 to 3% at 1.5 T (or smaller)
•• Too small to see in individual imagesToo small to see in individual images
•• Always considering differences or timeAlways considering differences or time--course changes course changes in image intensityin image intensity
•• Response is delayed and quite slow (~10 seconds)Response is delayed and quite slow (~10 seconds)•• Extracting temporal information is tricky, but possibleExtracting temporal information is tricky, but possible
•• Even short events have a rather long responseEven short events have a rather long response
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Blood Oxygen Level Dependent Blood Oxygen Level Dependent (BOLD) Contrast Activation(BOLD) Contrast Activation
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TimeTime--Course Response in fMRICourse Response in fMRI•• Brief neuronal events can Brief neuronal events can
elicit a (positive) blood elicit a (positive) blood flow and oxygenation flow and oxygenation response.response.
•• Reponses to events as Reponses to events as brief as 50 ms have been brief as 50 ms have been recorded.recorded.
Functional MRI response to a Functional MRI response to a visual stimulus of duration 2svisual stimulus of duration 2s
Start of Start of EventEvent
SlowerSlowerNegativeNegativeResponseResponse
Rise andRise andFall in ~10 sFall in ~10 s
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Response to periodic flashes of lightResponse to periodic flashes of light
Processed ImageProcessed Image Anatomic ImageAnatomic Image
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Typical Functional Image VolumeTypical Functional Image Volume
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fMRI Experiment Stages: PrepfMRI Experiment Stages: Prep
1) Prepare subject• Consent form• Safety screening• Instructions
2) Shimming• putting body in magnetic field makes it non-uniform• adjust 3 orthogonal weak magnets to make magnetic field as homogenous as
possible
3) SagittalsTake images along the midline to use to plan slices
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Slice Thicknesse.g., 6 mm
Number of Slicese.g., 10
SAGITTAL SLICE IN-PLANE SLICE
Field of View (FOV)e.g., 19.2 cm
VOXEL(Volumetric Pixel)
3 mm
3 mm6 mm
Slice TerminologySlice Terminology
Matrix Sizee.g., 64 x 64
In-plane resolutione.g., 192 mm / 64
= 3 mm
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fMRI Experiment Stages: fMRI Experiment Stages: FunctionalsFunctionals5) Take functional (T2*) images
• images are indirectly related to neural activity• usually low resolution images (3x3x5 mm)• all slices at one time = a volume (sometimes also called an image)• sample many volumes (time points) (e.g., 1 volume every 2 seconds for 150
volumes = 300 sec = 5 minutes)• 4D data: 3 spatial, 1 temporal
first volume(2 sec to acquire)
…
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MRI vs. fMRI
neural activity blood oxygen fMRI signal
MRI fMRI
one image
many images(e.g., every 2 sec for 5 mins)
high resolution(1 mm)
low resolution(~3 mm but can be better)
fMRIBlood Oxygenation Level Dependent (BOLD) signal
indirect measure of neural activity
…
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Detection/Detection/EstimationEstimation
fMRI process chain
RegistrationRegistrationFunctional ImagesFunctional Images
Threshold/Threshold/OverlayOverlay
Phase FixPhase Fix
TimeTime 11 22 33 …… 750 750 (secs)(secs)
11 2233
0s0s .66s.66s.33s.33s
11 22
y Xβ e
NormalizationNormalization
11 22
10
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Statistical Mapsuperimposed on
anatomical MRI image
~2s
Functional images
Time
Condition 1
Condition 2 ...
~ 5 min
Time
fMRISignal
(% change)
ROI Time Course
Condition
Activation StatisticsActivation Statistics
Region of interest (ROI)
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% s
igna
l cha
nge
images
Stimulation protocols in fMRI
baseline rest
stimulationhaemodynamic
response function
time courseof activation
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Statistical Maps & Time CoursesStatistical Maps & Time Courses
Use stat maps to pick regions
Then extract the time course
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2D 2D 3D3D
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Design Jargon: RunsDesign Jargon: Runs
run (or scan): one continuous period of fMRI scanning (~5-7 min) session: all of the scans collected from one subject in one day
experiment: a set of conditions you want to compare to each othercondition: one set of stimuli or one task
4 stimulus conditions+ 1 baseline condition (fixation)
A session consists of one or more experiments.Each experiment consists of several (e.g., 1-8) runsMore runs/expt are needed when SNR is low or the effect is weak.Thus each session consists of numerous (e.g., 5-20) runs (e.g., 0.5 – 3 hours)
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Design Jargon: Paradigm or ProtocolDesign Jargon: Paradigm or Protocol
paradigm (or protocol): the set of conditions and their order used in a particular run
Time
volume #1(time = 0)
volume #105(time = 105 vol x 2 sec/vol = 210 sec = 3:30)
runepoch: one instance of a condition
first “objects right” epochsecond “objects right” epoch
epoch 8 vol x 2 sec/vol = 16 sec
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Susceptibility in MRSusceptibility in MR
The good.The good.
The bad.The bad.
The ugly.The ugly.
All susceptibility effects increase with Bo field
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Susceptibility in Temporal LobesSusceptibility in Temporal Lobes
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What is the source of susceptibility?
The magnet has a spatially uniform field but your head is magnetic…
1) deoxyHeme is paramagnetic
2) Water is diamagnetic ( = -10-5)
3) Air is paramagnetic ( = 4x10-6)
Pattern of B field outside magnetic object in a uniform
field…
Bo
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Bo
Ping-pong ball in H20:Field maps (TE = 5ms), black lines spaced by 0.024G (0.8ppm at 3T)
1.5T 3T
Susceptibility effects occur near magnetically Susceptibility effects occur near magnetically disdis--similar materialssimilar materials
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Bo map in head: it’s the air tissue interface…
Sagittal Bo field maps at 3T
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Other Sources of Susceptibility You Should Other Sources of Susceptibility You Should Be Aware ofBe Aware of……
Those fillings might be a problem…
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Local susceptibility gradients:2 effects
• Local dephasing of the signal (signal loss) within a voxel, mainly from thru-plane gradients
• Local geometric distortions, (voxel location improperly reconstructed) mainly from local in-plane gradients (in PE direction).
Sagittal Bo field map at 3T
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Bandwidth is asymmetric in EPI(Distortion is 100x more in phase direction)
The phase error (and thus distortions) are in the phase encode direction.
=
kx
ky
t=0.005ms
t=0.5ms
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Susceptibility in EPI can give either a Susceptibility in EPI can give either a compression or expansioncompression or expansion
Altering the direction kspace is traversed causes either local compression or expansion.
choose your poison…
3T whole body gradients
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Susceptibility Causes Image DistortionSusceptibility Causes Image Distortion
Field near sinus
z
Echoplanar Image, encode time 1/BW
Encode time = 34, 26, 22, 17ms
3T head gradients
Use shortest possible encoding
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With fast gradients, add parallel imagingWith fast gradients, add parallel imaging
Acquisition: SM
AS
H
SE
NS
E
Reconstruction:
Folded datasets+
Coil sensitivity maps
Reduced k-space sampling
{
Folded images ineach receiver channel
FOVk
2
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3T MAGNETOM Allegra 3T MAGNETOM Allegra ss EPI PATss EPI PAT
MAGNETOM Allegra. Courtesy Bruker Medical and USA Instruments.4 channel tx/rx array coil
Single shotTE = 30 ms
with PAT x2192x128
with PAT x2128x128
with PAT x264x64
Conventional64x64
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•• Good shimming (first & second order)Good shimming (first & second order)•• Thinner slices (Drawback: Takes more to cover the brain)Thinner slices (Drawback: Takes more to cover the brain)•• Shorter TE (Drawback: BOLD contrast is optimized for TE = T2*locShorter TE (Drawback: BOLD contrast is optimized for TE = T2*local)al)•• ““ZZ--shimmingshimming”” Repeat measurement several times with an applied z Repeat measurement several times with an applied z
gradients that rewind the gradients that rewind the dephasingdephasing, Pick the right gradient afterward on a , Pick the right gradient afterward on a pixel by pixel basis. (Drawback: multi shot or longer encode). pixel by pixel basis. (Drawback: multi shot or longer encode). Yang et al. Yang et al. MRM 39 p402, 1998.MRM 39 p402, 1998.
•• Use special RF pulse with builtUse special RF pulse with built--in in prephasingprephasing in just the right places. in just the right places. (Drawback: long RF pulse, pre(Drawback: long RF pulse, pre--phasing differs from person to person) phasing differs from person to person) Glover et al. Proceed. ISMRM p298, 1998.Glover et al. Proceed. ISMRM p298, 1998.
•• The The ““mouth shimmouth shim”” paramagnetic material in roof of mouth. paramagnetic material in roof of mouth. Wilson, Wilson, JenkinsonJenkinson, , JezzardJezzard, Proceed. ISMRM p205, 2002., Proceed. ISMRM p205, 2002.
•• Distortion correction based on a measured field map (drawback: cDistortion correction based on a measured field map (drawback: cannot annot recover signal dropout or fully correct recover signal dropout or fully correct ““overlappingoverlapping”” intensities)intensities)
•• MultiMulti--shot imaging methods (drawback: more motion sensitive)shot imaging methods (drawback: more motion sensitive)•• Fancy pulse sequences (best to have local physicist): 180 degreeFancy pulse sequences (best to have local physicist): 180 degree
refocusing pulses to reverse distortion (GRASE)/Multiple refocusrefocusing pulses to reverse distortion (GRASE)/Multiple refocusing ing pulsespulses…… singlesingle--shot FSE, Ushot FSE, U--FlareFlare
What can you do?What can you do?
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SingleSingle--shot Gradient Echo EPIshot Gradient Echo EPI•• Parameters you can chooseParameters you can choose
•• TRTR
•• Slice thickness/gapSlice thickness/gap
•• Number of slices/slice acquisition orderNumber of slices/slice acquisition order
•• TETE
•• BandwidthBandwidth
•• Matrix sizeMatrix size
•• Field of viewField of view
•• Flip angleFlip angle
•• All of these parameters can be appropriately All of these parameters can be appropriately applied over a wide range of valuesapplied over a wide range of values
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TR (repetition time)TR (repetition time)•• Determines how much magnetization is allowed to Determines how much magnetization is allowed to
recover before it is knocked over again by the next rf recover before it is knocked over again by the next rf pulsepulse
•• From a pure signal strength perspective, waiting for From a pure signal strength perspective, waiting for very long very long TRTR’’ss (5 seconds +) allows for maximal (5 seconds +) allows for maximal signalsignal--toto--noise (SNR)noise (SNR)
•• Noise is MR dominated by physiologic noise (not Noise is MR dominated by physiologic noise (not thermal noise)thermal noise)
•• Requires many images in both conditions to reliably Requires many images in both conditions to reliably distinguish activation (which requires shorter distinguish activation (which requires shorter TRTR’’ss))
•• fMRI can be performed as fast as TR=100msfMRI can be performed as fast as TR=100ms•• Bottom line: use as short a TR as you canBottom line: use as short a TR as you can
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Flip AngleFlip Angle•• A given flip angle will maximize the SNR (Ernst A given flip angle will maximize the SNR (Ernst
Angle)Angle)……at long at long TRTR’’ss (> 3s) this is 90 degrees(> 3s) this is 90 degrees
•• This angle is dependent upon the TRThis angle is dependent upon the TR
•• Incorrect angles may sensitize your BOLD scans to inIncorrect angles may sensitize your BOLD scans to in--flow artifacts (bad) flow artifacts (bad) [Lu et al, NeuroImage 17, 943[Lu et al, NeuroImage 17, 943––955 (2002)]955 (2002)]
•• Bottom line: For TR of 1Bottom line: For TR of 1--2s, a flip angle of around 602s, a flip angle of around 60--70 degrees is optimal70 degrees is optimal
11cos exp /TR T
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Number of slicesNumber of slices•• Separate slices in EPI are typically squeezed into a TR Separate slices in EPI are typically squeezed into a TR
intervalinterval
•• Many factors influence # of slices that fit in a TRMany factors influence # of slices that fit in a TR•• Length of TRLength of TR
•• TE (determines center of blue box)TE (determines center of blue box)
•• Matrix size (determines length of blue box)Matrix size (determines length of blue box)
•• Bandwidth (determines length of blue box)Bandwidth (determines length of blue box)
•• Bottom line: collect as many slices as you canBottom line: collect as many slices as you can
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So farSo far•• Long TR maximized SNRLong TR maximized SNR•• Short TR maximizes fMRI statsShort TR maximizes fMRI stats•• Long TR provides many slicesLong TR provides many slices•• Short TR provides few slicesShort TR provides few slices
•• The above suggests imaging only brain regions of The above suggests imaging only brain regions of interest (to minimize slices)interest (to minimize slices)
•• But processing decisions also play a roleBut processing decisions also play a role•• Whole brain data is much easier to spatially normalizeWhole brain data is much easier to spatially normalize•• Motion correction works best with thin slicesMotion correction works best with thin slices•• In general In general TRTR’’ss between 1s and 2s are not too badbetween 1s and 2s are not too bad
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Slice ThicknessSlice Thickness•• SNR in MRI is proportional to voxel volume (thinner SNR in MRI is proportional to voxel volume (thinner
slices slices --> less SNR)> less SNR)
•• Thinner slices reduces partial volume effectsThinner slices reduces partial volume effects
•• Thinner slices reduces throughThinner slices reduces through--plan plan dephasingdephasing
•• What is the size of the structure of interest?What is the size of the structure of interest?
•• Isotropic voxel size is preferredIsotropic voxel size is preferred
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TE (echo time)TE (echo time)•• Optimum TE is shorter at high field (say 30ms at 3T Optimum TE is shorter at high field (say 30ms at 3T
versus 50ms at 1.5T)versus 50ms at 1.5T)
•• Shorter TE reduces signal loss due to field Shorter TE reduces signal loss due to field inhomogeneities, but also reduces BOLD effect inhomogeneities, but also reduces BOLD effect
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BandwidthBandwidth•• Rate at which points are sampled (the echoes are Rate at which points are sampled (the echoes are
digitized)digitized)
•• High bandwidth implies a high sampling rateHigh bandwidth implies a high sampling rate•• Sampling of the order of 128 kHzSampling of the order of 128 kHz
•• 128kHz/64matrix = 2000Hz/pixel128kHz/64matrix = 2000Hz/pixel
•• Noise is proportional to sampling rateNoise is proportional to sampling rate
•• High bandwidth means faster data acquisition (and High bandwidth means faster data acquisition (and more slices can be acquired, with less T2 blurring)more slices can be acquired, with less T2 blurring)
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Matrix SizeMatrix Size•• Matrix size impact everythingMatrix size impact everything
•• Increasing matrix size decreases voxel size and thus SNRIncreasing matrix size decreases voxel size and thus SNR
•• Increasing matrix and FOV maintains constant voxel size, but Increasing matrix and FOV maintains constant voxel size, but increases N and therefore increases SNRincreases N and therefore increases SNR
•• IntravoxelIntravoxel dephasingdephasing reduced somewhat with smaller voxels reduced somewhat with smaller voxels (bigger matrix)(bigger matrix)
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Field of View (FOV)Field of View (FOV)•• Voxel size determined by field of view and matrix sizeVoxel size determined by field of view and matrix size
•• FOV=200mm/64 matrix = 3.125mm voxel dimensionFOV=200mm/64 matrix = 3.125mm voxel dimension
•• Recall SNR proportional to voxel volumeRecall SNR proportional to voxel volume
x
x
FOVx
N y
y
FOVy
N