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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 l.jpg
BOLD Imaging at 7T

Mark Elliott

CfN Symposium 4/9/2008

Overview l.jpg

  • Introduction

  • Technical/Safety Issues

  • Contrast Changes

  • Benefits/Challenges

Introduction l.jpg

  • 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


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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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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)



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

Improved bold cnr11 l.jpg

high res

low res

Improved BOLD CNR

Use greater BOLD CNR to increase resolution

Spin-echo EPI

low res



high res


T. Duong et al, Magnetic Resonance in Medicine 48:589–593 (2002)

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Optimal TE Decreases with B0

TE (msec)



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)

Se bold at 7t l.jpg

  • 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

    B1 inhomogeneity l.jpg
    B1 Inhomogeneity

    • More complicated coil designs – phased array coils

    • Inhomogeneity corrections – B1 mapping methods


    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