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BOLD Imaging at 7T

BOLD Imaging at 7T. Mark Elliott C f N 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

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BOLD Imaging at 7T

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  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  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

  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. Contrast Changes Tissue relaxation properties depend on field strength 3T7T

  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 (note different scales) 1.5T 7T Gizewski et al, NeuroImage 37 (2007) 761–768

  11. high res low res Improved BOLD CNR Use greater BOLD CNR to increase resolution Spin-echo EPI 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” courtesy of Chen Lin Yacoub et al

  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

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