Bruce Fischl MGH ATHINOULA A. MARTINOS CENTER Harvard Medical School MIT CSAIL/HST some open issues...
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Transcript of Bruce Fischl MGH ATHINOULA A. MARTINOS CENTER Harvard Medical School MIT CSAIL/HST some open issues...
![Page 1: Bruce Fischl MGH ATHINOULA A. MARTINOS CENTER Harvard Medical School MIT CSAIL/HST some open issues in functional MRI (thanks to Larry Wald for almost.](https://reader033.fdocuments.in/reader033/viewer/2022051316/56649d0c5503460f949dff9f/html5/thumbnails/1.jpg)
Bruce Fischl
MGH ATHINOULA A. MARTINOS CENTERHarvard Medical School
MIT CSAIL/HST
some open issues in functional MRI(thanks to Larry Wald for almost all these slides)
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Integrating Function and Structure.
Local functional organization of cortex is largely 2-dimensional!
From (Sereno et al, 1995, Science).
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Some hot topics/open questions:
1. EPI distortions.
2. Large N arrays/parallel acquisition and
High field (or ultra-high field).
3. Open issues.
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EPI distortions: change with TE.
3T, TE = 21, 30, 40, 50, 60ms
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EPI distortions: change with slice thickness.
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EPI distortions: other factors
1. Scale linearly with field strength.
2. Change with changing shim.
3. Bone/air interface particularly problematic.
4. Are inversely proportional to bandwidth.
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Some hot topics/open questions:
1. EPI distortions.
2. Large N arrays/parallel acquisition and
High field (or ultra-high field).
3. Open issues.
![Page 8: Bruce Fischl MGH ATHINOULA A. MARTINOS CENTER Harvard Medical School MIT CSAIL/HST some open issues in functional MRI (thanks to Larry Wald for almost.](https://reader033.fdocuments.in/reader033/viewer/2022051316/56649d0c5503460f949dff9f/html5/thumbnails/8.jpg)
Large N Arrays
1. The sensitivity of a coil generally scales inversely
with its area.
2. The sensitivity of a coil falls off with depth, with the
rate of falloff inversely proportional to the coil
diameter.
How to get uniformly high sensitivity?
Tile the head with lots of little coils!
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Two ways to improve sensitivity in the brain.
Big magnetSmall coils
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Two ways to improve sensitivity in the brain.
Big magnet Small coils
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MGH array development
32 ch. at 3T
90 ch. at 1.5T
32 ch. at 7T
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SNR Maps – increased sensitivity everywhere
23 Channel 8 Channel CP Head
0
10
20
30
40
5090 Channel
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Acceleration
Arrays contain lots of redundant information
(since each coil sees the entire brain, albeit
with variable SNR)
As usual in MRI, can tradeoff time and SNR –
acceleration to increase EPI bandwidth and
decrease distortion.
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1/FOV
water
ky
kx
Under-sampled kspace
Increasing Bandwidth: accelerate the EPI by leaving out every other line…
1/2 FOV, from under-sampled kspace
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Two ways to get to the unfolded image…
FFTkspace, every other line (under-sampled)
Folded, but many
FFT
SMASH, GRAPPA SENSE
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EPI distortions: acceleration
4x acceleration at 1.5T vs. no acceleration
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3D Flash with 16x accel.
1mm x 1mm x 2mm, acquisition time = 23secCourtesy: Mathias Nittka, Siemens
TR = 12TE = 4.7FA = 15 degBW = 130
3T: 32 channel
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What else can be done with SNR?
Can also trade SNR for resolution, although the price is high!
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MGH 3T
32 channel
3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
24
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
28
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
32
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
36
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
40
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
44
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
48
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
52
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
56
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
60
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
64
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3T MP-RAGE380um x 380um x 1mm, 7 scans of 9 minutes eachmotion correctedTI=900ms, TR/TE/flip = 2250/4.35/9deg
MGH 3T
32 channel
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MGH 3T
32 channel
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Single shot GRE EPI1.0mm isotropic, 192x192, TE= 30msR=2 GRAPPA
MGH 3T
32 channel
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1mm Res. Diffusion MRIiPAT=2 iPAT=3
iPAT=5iPAT=4
Product EPI
b = 1000 s/mm2
256x256 matrix
FoV = 256 mm
Thick = 2.0 mm
TR = 3200 ms
TE (iPAT 2) = 112 ms
TE (iPAT 3) = 94 ms
TE (iPAT 4) = 87 ms
TE (iPAT 5) = 87 ms
Shots 12-30
Courtesy Michael Zwanger & Gunnar Kruger
3T 32 channel
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Courtesy: G. Sorensen and R. Wang, Thomas Benner, MGH Martinos Center
2mm isotropic DTI,whole brain, 7 minutes
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7T array coil750um isotropic resolution
7T High Resol. fMRI w/ BOSS method
results from C. Miller, Univ. Oxford and C. Wiggins
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1. What to do about the veins?
2. How to accurately align high res functional data to structurals?
3. Non-Fourier reconstructions to account for other distortions?
4. What to do about physiological noise?
5. How to optimally combine different echoes?
Open Questions for High-Field
High resolution imaging.
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7 Tesla: Dielectric Resonance within head
Intensity inhomogeneity in transmit B1 field (changes contrast!)
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How to get rid of the veins?
What is the true limit of the neural resolution of fMRI?
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Larry WaldGraham Wiggins Andreas PotthastChris Wiggins Franz SchmittChristina Triantafyllou Gunnar KruegerGreg Sorensen Mattias NitkaThomas Benner many othersmany others
MGH Siemens
Acknowledgements