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BOLD Imaging at 7T Mark Elliott C f N Symposium 4/9/2008.

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Presentation on theme: "BOLD Imaging at 7T Mark Elliott C f N Symposium 4/9/2008."— Presentation transcript:

1 BOLD Imaging at 7T Mark Elliott C f N Symposium 4/9/2008

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

3 Introduction 3T MRI whole body – FDA approved in T – 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 0 2 Force  B 0  B 0 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)  =  B 0 Tissue conductivity  sqrt(  ) wavelength effects  localized heating

6 Contrast Changes Tissue relaxation properties depend on field strength T1 (msec) T2 (msec) Gray matter   59 White matter   54 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 TSE, 7 min scan, 0.4x0.4x3mm MR signal increases with frequency & field strength! from Siemens Medical Systems

9 SNR versus Field Strength 7T: SNR ~ linear with B 0, but BOLD  B 0 2 (  R2’  B 0 )

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

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

12 Optimal TE Decreases with B 0 Yacoub et al, Magnetic Resonance in Medicine 45:588–594 (2001) 4T TE (msec) 7T TE = 22msTE = 34ms

13 Increased Specificity of SE-BOLD from S.P. Lee et al, (2003) Intravascular BOLD fraction Intravascular GE-BOLD signal greatest in venous compartment Low spatial specificity to neuronal activity T2 & T2* both decrease quadratically with B 0 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

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

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

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

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 (3 rd party) No Body coil fMRI stimulus capabilities FORP, projector, Avotec audio

19 Done


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