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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

Mark Elliott

CfN Symposium 4/9/2008


Overview l.jpg
Overview

  • Introduction

  • Technical/Safety Issues

  • Contrast Changes

  • Benefits/Challenges


Introduction l.jpg
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


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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


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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


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Contrast Changes

Tissue relaxation properties depend on field strength

3T7T


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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


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Benefits/Challenges

MR signal increases with frequency & field strength!

TSE, 7 min scan, 0.4x0.4x3mm

from Siemens Medical Systems


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SNR versus Field Strength

7T: SNR ~ linear with B0, but BOLD  B02 (R2’  B0)


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Improved BOLD CNR

Use greater BOLD CNR to increase sensitivity

(note different scales)

1.5T

7T

Gizewski et al, NeuroImage 37 (2007) 761–768


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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)


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Optimal TE Decreases with B0

TE (msec)

4T

7T

TE = 22ms

TE = 34ms

Yacoub et al, Magnetic Resonance in Medicine 45:588–594 (2001)


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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)


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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)


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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


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    B1 Inhomogeneity

    • More complicated coil designs – phased array coils

    • Inhomogeneity corrections – B1 mapping methods

    7T

    Uncorrected Corrected

    courtesy L. Wald


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    SNR Gains for sMRI

    TSE, 512x760 matrix, (0.27x0.27x2mm), 9 min scan

    line of Gennari

    courtesy L. Wald


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    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



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