Magnetic resonance in food science : defining food by magnetic resonance
Improved Functional Magnetic Resonance Imaging at 4.0 T
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Transcript of Improved Functional Magnetic Resonance Imaging at 4.0 T
Improved Functional Magnetic Resonance
Imaging at 4.0 T
Kimberly BrewerPhD External Defence – Physics and Atmospheric ScienceMarch 18, 2010
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MRI and Relaxation
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• R2 - transverse signal decay rate due to spin-spin interactions (R2 = 1/T2)
• R2’ - transverse relaxation rate from local field inhomogeneities (R2’ = 1/T2’)
• R2* = R2 + R2’
B0
Functional MRI (FMRI) - BOLDBOLD – Blood oxygen level-dependent
◦ Deoxy Hb is paramagnetic, oxy Hb is diamagnetic
◦ More deoxy Hb the MRI signal◦ After stimulus, ratio of oxy Hb/deoxy Hb ,
causing in the MRI signalA R2*-weighted sequence is generally
used for fMRIAt high fields, BOLD CNR increases
Susceptibility Field Gradients (SFGs)
Occur in regions where the magnetic susceptibility changes rapidly◦ E.g. Inferior temporal, orbital
frontalThe large magnetic field
gradients cause rapid dephasingCauses signal loss and other
artifacts◦ No fMRI activation in these regions,
or activation is displacedEffects are worse at higher
magnetic fields
Traditional
“Ideal”
K-Space and Images
Signal collected as frequency and phase information – build representation of image in k-space
Image is complex – has both magnitude and phase information
K-space traversal depends on gradient patternsUse rectilinear or spiral trajectories
FT
Spiral-In vs Spiral-Out
Spiral-In TE = 30 ms
Spiral-Out TE = 19 ms1. Glover and Law, Magn Reson Med 46:515-522 (2001)
TE
“Ideal” Sequence for SFG regions
Minimal apparent geometric distortionMaximum signal-to-noise ratio (SNR)Optimal R2’-weighting for maximum
BOLD contrast-to-noise ratio (CNR)High specificity to extravascular sources
(less sensitivity to large vessels)
TE
TE* TE* TE*
Asymmetric Spin-Echo (ASE) Spiral
Asymmetric Spin-Echo (ASE) Spiral
Spiral-Out ASE Image 1 ASE Image 2
SNR Results
8 subjects
fMRI Results
Spiral-Out
ASE Image 1
ASE Image 2
ASE Image 3
30s breath-holding task, 5 subjects
Percent Signal Change, SNR and CNR
ASE Spiral & SpecificitySpin-echo more specific to tissue compared to
vessel at high magnetic field strengths◦ The T2 of blood at high fields is quite short◦ At TE > 65 ms (4 T), less than 25% of spin-echo
fMRI signal is intravascularIncreasing R2-weighting in later ASE spiral
images may lead to specificity improvements◦ Common TE/TE* combinations (ie. 60-70/30 ms)
- third ASE image has effective R2-weighting = a spin-echo spiral-in at TE = 90-100 ms.
Determine where ASE spiral activation is located◦ Compare to pure gradient-echo and spin-echo
FMRI Results
20s alternating checkerboard task, 12 subjects, 2mm in-plane resolution, 3mm thick
Average % Signal Change (ΔS/S) in Tissue and Vasculature
20s alternating checkerboard task task, 12 subjects
Sensitivity vs SpecificityLater ASE images similar to spin-echo images
◦ In appearance and in % signal changeHave not yet proved that later images are more
specific◦ Need a better metric – Use an individualized
specificity analysis with venogramBased off ROC curves - function of false positive
rate (FPR) ◦ Number of false positives – activation on veins; Number
of true negatives – voxels in vessels with no activation)◦ specificity = 1 – FPR◦ Generate specificity curves as a function of
varying z-thresholds – the faster a curve reaches a value of 1.0, the more specific it is to tissue compared to vessel
Specificity Curve
FPR = 50%
FPR = 0%
12 subjects
Conclusions - SpecificityThe 2nd ASE image may be the
most useful◦Has stronger activation (and more
active voxels) ◦The specificity curve is not
significantly different than the 3rd image
◦Could help improve temporal resolution
◦May be able to change TE/TE* to improve intravascular suppression
ConclusionsDeveloped a novel pulse sequence, ASE
spiral, that is effective at recovering signal lost in SFG regions while maintaining significant BOLD contrast
Determined that individual ASE spiral images have varying degrees of sensitivity and specificity to fMRI activation ◦The 2nd and 3rd ASE images are more
specific to extravascular sources than either spiral-in or spiral-out
Acknowlegements Dr. Steven Beyea Dr. Chris Bowen Dr. Ryan D’Arcy Careesa Liu Sujoy Ghosh-Hajra Dr. Martyn Klassen Janet Marshall
James Rioux Lindsay Cherpak Tynan Stevens Jodie Gawryluk Erin Mazerolle Connie Adsett Ahmed Elkady Everyone at IBD
Atlantic…Walter C. Sumner Foundation
Questions?
Future Directions – Current ImpactASE spiral is currently being used to study white
matter fMRI◦ Collaborators have found that ASE spiral is more
sensitive to the detection of activation located in white matter (corpus callosum) Increase from 21% to 100% of subjects with
activation◦ Also saw increasing ΔS/S with increasing R2-
weightingASE spiral is currently being used for a temporal
lobe epilepsy study◦ Has successfully elicited activation throughout the
temporal cortex in several subjects and is insensitive to signal loss around metal clips found in post-surgical patients
Future Directions Further spiral-in/spiral-out simulations
◦ Using a realistic head model will give more accurate signal displacement information
Comprehensive study is currently be doing to compare ASE spiral and other SFG recovery methods (spiral-in/out & spiral-in/in) to traditional (EPI & spiral) and non-BOLD (spin-echo spiral-in/out and FAIR) fMRI techniques◦ Uses a task to elicit activation in the temporal lobe◦ Will determine the effectiveness of signal recovery using a
cognitive task Monte Carlo simulations would be useful for modeling the
specific contributions (tissue vs vasculature) occurring in both grey and white matter for each of the individual ASE spiral images
Also need to investigate different image addition methods◦ May be able to gain both specificity and sensitivity benefits in
post-processing
Conclusions - Specificity Later ASE spiral images have activation patterns
similar to spin-echo images ΔS/S increases with increasing R2-weighting in
tissue but remains constant in vasculature The 2nd and 3rd ASE spiral images are more
specific than a pure gradient-echo, but less specific than spin-echo
The 2nd ASE image may be the most useful◦ Has stronger activation (and more active voxels) ◦ The specificity curve is not significantly different than
the 3rd image◦ Could help improve temporal resolution◦ May be able to change TE/TE* to improve intravascular
suppression
Conclusions – ASE spiralEach individual image has reduced
apparent geometric distortion and minimal signal loss
SNR decreases with increasing R2-weighting & % signal change increases to compensate◦Each image has equivalent CNR
Combining images gives higher SNR and has more active voxels
SNR Results
fMRI Results
ASE Spiral vs Spiral-Out8 healthy adults (4 males, 4 females)30 s breath-holding task
◦3 subjects were excluded from fMRI resultsTR = 3 s, 13 slice (5 mm, gap 0.5 mm)64 x 64 (240 x 240 mm) resolutionSpiral-out: TE = 25 msASE spiral: TE* = 25 ms, TE = 70 msMultiple images were combined with equal
weighting
Z-shim Asymmetric Spin-Echo Spiral
Can use unique z-shim gradient (in red) for each individual ASE image
Z-Shim Automated Routines Prescan-based routines – Optimal
combination must have sufficient SNR and large number of recovered voxels
1. MIP-based routine - Images are combined with a maximum intensity projection (MIP) in routine
2. SS-based routine – Images are combined with a sum-of-squares (SS) in routine
B0 field routine – Developed by Truong and Song (2008)
◦ Calculates offsets from an initial field map and calculates the gradients necessary to provide opposing phase twist
* Truong et al., Magn Reson Med 59:221-227 (2008)
Z-Shim ASE Spiral vs ASE Spiral8 healthy adults (4 males, 4 females)24 s breath-holding task
◦1 subject was excluded from fMRI results
TR = 4 s, 18 slice (5 mm, gap 0.5 mm)64 x 64 (240 x 240 mm) resolutionZ-shim ASE spiral & ASE spiral: TE* =
25 ms, TE = 70 msImages were combined with MIP or SS
ASE Spiral Specificity Experiment12 healthy adults (3 males, 9 females)20 s alternating checkerboard task
◦ Alternating at 8 HzTR = 2 s (4-shot), 4 slices (3 mm, gap 0.5 mm)
◦ Slices centred and aligned along calcarine sulcus128 x 128 (240 x 240 mm) – 1 mm in-plane
resolutionSpiral-in/out: TE = 30 msSpin-echo spiral-in/out: TE = 105 msASE spiral: TE* = 30 ms, TE = 75 msVenogram: 256 x 256, TE = 30 ms – used for
delineation of vessels
ASE Spiral Specificity Experiment12 healthy adults (3 males, 9 females)20 s alternating checkerboard task
◦ Alternating at 8 Hz4 slices (3 mm)
◦ Slices centred and aligned along calcarine sulcus
2 mm in-plane resolutionSequences: Spiral-in/out, spin-echo
spiral-in/out, ASE spiralVenogram (1mm in-plane resolution) – used
for delineation of vessels