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 B0B0 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
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
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 1.5T 7T (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 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)
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 Intravascular BOLD fraction T2 & T2* effects versus vessel size from Principles of Functional MRI, Seong-Gi Kim 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” “Center brightening” Yacoub et al 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