Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations

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

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

2 Overview Introduction Technical/Safety Issues Contrast Changes

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

Download ppt "BOLD Imaging at 7T Mark Elliott CfN Symposium 4/9/2008."

Similar presentations

Ads by Google