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Improved Functional Magnetic Resonance Imaging at 4.0 T Kimberly Brewer PhD External Defence – Physics and Atmospheric Science March 18, 2010.

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Presentation on theme: "Improved Functional Magnetic Resonance Imaging at 4.0 T Kimberly Brewer PhD External Defence – Physics and Atmospheric Science March 18, 2010."— Presentation transcript:

1 Improved Functional Magnetic Resonance Imaging at 4.0 T Kimberly Brewer PhD External Defence – Physics and Atmospheric Science March 18, 2010

2 x’ y’ z’ M MRI and Relaxation x’ y’ z’ x’ y’ z’ M 90 o R 2 - transverse signal decay rate due to spin-spin interactions (R 2 = 1/T 2 ) R 2 ’ - transverse relaxation rate from local field inhomogeneities (R 2 ’ = 1/T 2 ’) R 2 * = R 2 + R 2 ’ B0B0

3 Functional MRI (FMRI) - BOLD BOLD – 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 signal A R 2 *-weighted sequence is generally used for fMRI At high fields, BOLD CNR increases

4 Susceptibility Field Gradients (SFGs) Occur in regions where the magnetic susceptibility changes rapidly ◦ E.g. Inferior temporal, orbital frontal The large magnetic field gradients cause rapid dephasing Causes signal loss and other artifacts ◦ No fMRI activation in these regions, or activation is displaced Effects are worse at higher magnetic fields Traditional “Ideal”

5 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 patterns Use rectilinear or spiral trajectories FT

6 Spiral-In vs Spiral-Out Spiral-In TE = 30 ms Spiral-Out TE = 19 ms 1. Glover and Law, Magn Reson Med 46: (2001) TE

7 “Ideal” Sequence for SFG regions Minimal apparent geometric distortion Maximum signal-to-noise ratio (SNR) Optimal R 2 ’-weighting for maximum BOLD contrast-to-noise ratio (CNR) High specificity to extravascular sources (less sensitivity to large vessels)

8 TE TE* Asymmetric Spin-Echo (ASE) Spiral

9 Spiral-Out ASE Image 1ASE Image 2

10

11 SNR Results 8 subjects

12 fMRI Results Spiral-Out ASE Image 1 ASE Image 2 ASE Image 3 30s breath-holding task, 5 subjects

13 Percent Signal Change, SNR and CNR

14 ASE Spiral & Specificity Spin-echo more specific to tissue compared to vessel at high magnetic field strengths ◦ The T 2 of blood at high fields is quite short ◦ At TE > 65 ms (4 T), less than 25% of spin-echo fMRI signal is intravascular Increasing R 2 -weighting in later ASE spiral images may lead to specificity improvements ◦ Common TE/TE* combinations (ie /30 ms) - third ASE image has effective R 2 -weighting = a spin- echo spiral-in at TE = ms. Determine where ASE spiral activation is located ◦ Compare to pure gradient-echo and spin-echo

15 FMRI Results 20s alternating checkerboard task, 12 subjects, 2mm in-plane resolution, 3mm thick

16 Average % Signal Change ( Δ S/S) in Tissue and Vasculature 20s alternating checkerboard task task, 12 subjects

17 Sensitivity vs Specificity Later ASE images similar to spin-echo images ◦ In appearance and in % signal change Have not yet proved that later images are more specific ◦ Need a better metric – Use an individualized specificity analysis with venogram Based 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

18 Specificity Curve

19 FPR = 50% FPR = 0% 12 subjects

20 Conclusions - Specificity The 2 nd ASE image may be the most useful ◦ Has stronger activation (and more active voxels) ◦ The specificity curve is not significantly different than the 3 rd image ◦ Could help improve temporal resolution ◦ May be able to change TE/TE* to improve intravascular suppression

21 Conclusions Developed 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 2 nd and 3 rd ASE images are more specific to extravascular sources than either spiral-in or spiral-out

22 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

23 Questions?

24 Future Directions – Current Impact ASE 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 Δ S/S with increasing R 2 -weighting ASE 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

25 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

26 Conclusions - Specificity Later ASE spiral images have activation patterns similar to spin-echo images Δ S/S increases with increasing R 2 -weighting in tissue but remains constant in vasculature The 2 nd and 3 rd ASE spiral images are more specific than a pure gradient-echo, but less specific than spin-echo The 2 nd ASE image may be the most useful ◦ Has stronger activation (and more active voxels) ◦ The specificity curve is not significantly different than the 3 rd image ◦ Could help improve temporal resolution ◦ May be able to change TE/TE* to improve intravascular suppression

27 Conclusions – ASE spiral Each individual image has reduced apparent geometric distortion and minimal signal loss SNR decreases with increasing R 2 -weighting & % signal change increases to compensate ◦ Each image has equivalent CNR Combining images gives higher SNR and has more active voxels

28

29 SNR Results

30 fMRI Results

31 ASE Spiral vs Spiral-Out 8 healthy adults (4 males, 4 females) 30 s breath-holding task ◦ 3 subjects were excluded from fMRI results TR = 3 s, 13 slice (5 mm, gap 0.5 mm) 64 x 64 (240 x 240 mm) resolution Spiral-out: TE = 25 ms ASE spiral: TE* = 25 ms, TE = 70 ms Multiple images were combined with equal weighting

32 Z-shim Asymmetric Spin-Echo Spiral Can use unique z-shim gradient (in red) for each individual ASE image

33 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 B 0 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: (2008)

34 Z-Shim ASE Spiral vs ASE Spiral 8 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) resolution Z-shim ASE spiral & ASE spiral: TE* = 25 ms, TE = 70 ms Images were combined with MIP or SS

35

36 ASE Spiral Specificity Experiment 12 healthy adults (3 males, 9 females) 20 s alternating checkerboard task ◦ Alternating at 8 Hz TR = 2 s (4-shot), 4 slices (3 mm, gap 0.5 mm) ◦ Slices centred and aligned along calcarine sulcus 128 x 128 (240 x 240 mm) – 1 mm in-plane resolution Spiral-in/out: TE = 30 ms Spin-echo spiral-in/out: TE = 105 ms ASE spiral: TE* = 30 ms, TE = 75 ms Venogram: 256 x 256, TE = 30 ms – used for delineation of vessels

37 ASE Spiral Specificity Experiment 12 healthy adults (3 males, 9 females) 20 s alternating checkerboard task ◦ Alternating at 8 Hz 4 slices (3 mm) ◦ Slices centred and aligned along calcarine sulcus 2 mm in-plane resolution Sequences: Spiral-in/out, spin-echo spiral-in/out, ASE spiral Venogram (1mm in-plane resolution) – used for delineation of vessels


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