1. BOLD Imaging at 7T
Mark Elliott
CfN Symposium 4/9/2008

2. Overview Introduction Technical/Safety Issues Contrast Changes
Benefits/Challenges

3. Introduction 3T MRI whole body – FDA approved in 2002
4T – 9.4T whole body MRI currently in evaluation
only ~12 7T whole body MR worldwide
8 Siemens, 2 more being installed (Vienna, Penn)
Basic concept: More Tesla = more signal

4. Technical/Safety Issues
Cost & siting challenges
Larger fringe fields
Increased shielding: 500 tons for Penn 7T!
Increased flying object risk
Torque  B Force  B0B0
Compatibility of medical devices?
Magnetohydrodynamic effects
Conductive fluid flow (blood) in B field
“T-wave swelling” – ECG distortions
Increased blood pressure
< 0.2% at 10 T
No risk until ~18T

5. Technical/Safety Issues (continued)
Reported side effects of movement in 7T
Nausea
Vertigo
Headache
Symptoms dissipate rapidly
Reduced/avoided with slow table movement
Increased Specific Absorption Rate (SAR)
 = B0
Tissue conductivity  sqrt()
wavelength effects  localized heating

6. Tissue relaxation properties depend on field strength
Contrast Changes
Tissue relaxation properties depend on field strength
3T  7T
T1 (msec)
T2 (msec)
Gray matter
900  2000
80  59
White matter
750  1500
70  54

7. Contrast Changes (continued)
Increased T1:
Less signal per unit time (ignoring positive effects on signal)
Penalty for rapid imaging, longer TR for optimal SNR
Longer inversion times needed
IR (MPRAGE) and Perf (FLAIR)
Reduced flip-angles for GRE BOLD
Decreased T2/T2*:
Small decrease for T2, Large decrease for T2*
Spin-echo for BOLD coverage near tissue-air boundaries
Filtering effects on EPI
Parallel imaging or multi-shot reduces this affect

8. Benefits/Challenges
MR signal increases with frequency & field strength!
TSE, 7 min scan, 0.4x0.4x3mm
from Siemens Medical Systems

9. SNR versus Field Strength
7T: SNR ~ linear with B0, but BOLD  B02 (R2’  B0)

10. Improved BOLD CNR Use greater BOLD CNR to increase sensitivity 1.5T 7T
(note different scales)
1.5T 7T
Gizewski et al, NeuroImage 37 (2007) 761–768

11. Improved BOLD CNR Use greater BOLD CNR to increase resolution
Spin-echo EPI
high res
low res
low res
(1x1x2)
(overlap)
high res
(0.5x0.5x2)
T. Duong et al, Magnetic Resonance in Medicine 48:589–593 (2002)

12. Optimal TE Decreases with B0
TE (msec) 4T 7T
TE = 22ms
TE = 34ms
Yacoub et al, Magnetic Resonance in Medicine 45:588–594 (2001)

13. Increased Specificity of SE-BOLD
Intravascular GE-BOLD signal greatest in venous compartment
Low spatial specificity to neuronal activity
T2 & T2* both decrease quadratically with B0
3T: Blood T2  30ms 7T: Blood T2  7ms (60% sat.)
Negligible IV contribution to BOLD
SE-BOLD refocus EV BOLD around large vessels
Increased CNR compensates for decreased SE sensitivity
SE also reduces tissue-air signal dropout
Intravascular BOLD fraction
T2 & T2* effects versus vessel size
from Principles of Functional MRI, Seong-Gi Kim
from S.P. Lee et al, (2003)

14. SE BOLD at 7T Diffusion weighting eliminates vascular signal
Confirms reduced intra-vascular BOLD at 7T and up
Rat brain, 9.4T
from S.P. Lee et al, (2003)

15. Challenges at 7T Susceptibility effects scale with B0
- Increased B0 inhomogeneity – harder to shim
Dielectric and interference with higher frequencies
- Increased B1 inhomogeneity
“Penetration effects”
“Center brightening”
Yacoub et al
courtesy of Chen Lin

16. B1 Inhomogeneity More complicated coil designs – phased array coils
Inhomogeneity corrections – B1 mapping methods 7T
Uncorrected Corrected
courtesy L. Wald

17. SNR Gains for sMRI TSE, 512x760 matrix, (0.27x0.27x2mm), 9 min scan
line of Gennari
courtesy L. Wald

18. Capabilities at “HUP7” Siemens TIM, IDEA VB15
BOLD, Diffusion, Spectro, Multi-Nuclear
Siemens 12-channel head coil
18-channel proton/sodium head coil (3rd party)
No Body coil
fMRI stimulus capabilities
FORP, projector, Avotec audio

19. Done