BOLD Imaging at 7TBOLD Imaging at 7T

Mark Elliott

CfN Symposium 4/9/2008

OverviewOverview

• Introduction• Technical/Safety Issues• Contrast Changes• Benefits/Challenges

IntroductionIntroduction

• 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

Technical/Safety IssuesTechnical/Safety Issues• Cost & siting challenges

• Larger fringe fields• Increased shielding: 500 tons for Penn 7T!

• Increased flying object risk• Torque B0

2 Force B0B0

• 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

Technical/Safety Issues Technical/Safety Issues (continued)(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

Contrast ChangesContrast ChangesTissue relaxation properties depend on field strength

T1 (msec)T1 (msec) T2 (msec)T2 (msec)

Gray matterGray matter 900900

200020008080 59

White White mattermatter

750750

15007070 54

3T 7T

Contrast Changes Contrast Changes (continued)(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

Benefits/ChallengesBenefits/Challenges

TSE, 7 min scan, 0.4x0.4x3mm

MR signal increases with frequency & field strength!

from Siemens Medical Systems

SNR versus Field SNR versus Field StrengthStrength

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

Improved BOLD CNRImproved BOLD CNR

Use greater BOLD CNR to increase sensitivity

1.5T

7T

(note different scales)

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

Improved BOLD CNRImproved BOLD CNRUse 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 reslow res

Spin-echo EPI

Optimal TE Decreases Optimal TE Decreases with Bwith B00

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

4T

TE (msec)

7T

TE = 22ms TE = 34ms

Increased Specificity of SE-Increased Specificity of SE-BOLDBOLD

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 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

T2 & T2* effects versus vessel size

from Principles of Functional MRI, Seong-Gi Kim

SE BOLD at 7TSE BOLD at 7T

Rat brain, 9.4Tfrom S.P. Lee et al, (2003)

• Diffusion weighting eliminates vascular signal• Confirms reduced intra-vascular BOLD at 7T and up

Challenges at 7TChallenges 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”

B1 InhomogeneityB1 Inhomogeneity

• More complicated coil designs – phased array coils• Inhomogeneity corrections – B1 mapping methods

Uncorrected Correctedcourtesy L. Wald

7T

SNR Gains for sMRISNR Gains for sMRITSE, 512x760 matrix, (0.27x0.27x2mm), 9 min scan

line of Gennari

courtesy L. Wald

Capabilities at “HUP7”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

DoneDone