Transcript Mark Elliott, PhD - Center for Functional Neuroimaging

BOLD Imaging at 7T
Mark Elliott
CfN Symposium 4/9/2008
Overview
• Introduction
• Technical/Safety Issues
• Contrast Changes
• Benefits/Challenges
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
Technical/Safety Issues
• Cost & siting challenges
• Larger fringe fields
• Increased shielding: 500 tons for Penn 7T!
• Increased flying object risk
• Torque  B02
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
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
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
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
Benefits/Challenges
MR signal increases with frequency & field strength!
TSE, 7 min scan, 0.4x0.4x3mm
from Siemens Medical Systems
SNR versus Field Strength
7T: SNR ~ linear with B0, but BOLD  B02 (R2’  B0)
Improved BOLD CNR
Use greater BOLD CNR to increase sensitivity
(note different scales)
1.5T
7T
Gizewski et al, NeuroImage 37 (2007) 761–768
Improved BOLD CNR
Use greater BOLD CNR to increase resolution
Spin-echo EPI
low res
(1x1x2)
high res
low res
(overlap)
high res
(0.5x0.5x2)
T. Duong et al, Magnetic Resonance in Medicine 48:589–593 (2002)
Optimal TE Decreases with B0
TE (msec)
4T
7T
TE = 22ms
TE = 34ms
Yacoub et al, Magnetic Resonance in Medicine 45:588–594 (2001)
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
T2 & T2* effects versus vessel size
from Principles of Functional MRI, Seong-Gi Kim
Intravascular BOLD fraction
from S.P. Lee et al, (2003)
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)
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”
Yacoub et al
“Center brightening”
courtesy of Chen Lin
B1 Inhomogeneity
• More complicated coil designs – phased array coils
• Inhomogeneity corrections – B1 mapping methods
7T
Uncorrected
Corrected
courtesy L. Wald
SNR Gains for sMRI
TSE, 512x760 matrix, (0.27x0.27x2mm), 9 min scan
line of Gennari
courtesy L. Wald
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
Done