bruce fischl mgh athinoula a. martinos center harvard medical school mit csail/hst some open issues...
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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)
Integrating Function and Structure.
Local functional organization of cortex is largely 2-dimensional!
From (Sereno et al, 1995, Science).
Some hot topics/open questions:
1. EPI distortions.
2. Large N arrays/parallel acquisition and
High field (or ultra-high field).
3. Open issues.
EPI distortions: change with TE.
3T, TE = 21, 30, 40, 50, 60ms
EPI distortions: change with slice thickness.
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.
Some hot topics/open questions:
1. EPI distortions.
2. Large N arrays/parallel acquisition and
High field (or ultra-high field).
3. Open issues.
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!
Two ways to improve sensitivity in the brain.
Big magnetSmall coils
Two ways to improve sensitivity in the brain.
Big magnet Small coils
MGH array development
32 ch. at 3T
90 ch. at 1.5T
32 ch. at 7T
SNR Maps – increased sensitivity everywhere
23 Channel 8 Channel CP Head
0
10
20
30
40
5090 Channel
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.
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
Two ways to get to the unfolded image…
FFTkspace, every other line (under-sampled)
Folded, but many
FFT
SMASH, GRAPPA SENSE
EPI distortions: acceleration
4x acceleration at 1.5T vs. no acceleration
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
What else can be done with SNR?
Can also trade SNR for resolution, although the price is high!
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
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
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
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
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
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
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
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
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
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
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
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
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
MGH 3T
32 channel
Single shot GRE EPI1.0mm isotropic, 192x192, TE= 30msR=2 GRAPPA
MGH 3T
32 channel
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
Courtesy: G. Sorensen and R. Wang, Thomas Benner, MGH Martinos Center
2mm isotropic DTI,whole brain, 7 minutes
7T array coil750um isotropic resolution
7T High Resol. fMRI w/ BOSS method
results from C. Miller, Univ. Oxford and C. Wiggins
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.
7 Tesla: Dielectric Resonance within head
Intensity inhomogeneity in transmit B1 field (changes contrast!)
How to get rid of the veins?
What is the true limit of the neural resolution of fMRI?
Larry WaldGraham Wiggins Andreas PotthastChris Wiggins Franz SchmittChristina Triantafyllou Gunnar KruegerGreg Sorensen Mattias NitkaThomas Benner many othersmany others
MGH Siemens
Acknowledgements