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