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Arterial Spin Labeling at 7T - Double Edged Sword. Jiong-Jiong Wang, PhD University of Pennsylvania, Philadelphia, PA. Spin Labeling Strategies . ● Continuous ASL (CASL) vs. Pulsed ASL (PASL) Greater SNR, higher MT effect & SAR, technically challenging ● Pseudo-CASL (pCASL)

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arterial spin labeling at 7t double edged sword

Arterial Spin Labeling at 7T- Double Edged Sword

Jiong-Jiong Wang, PhD

University of Pennsylvania, Philadelphia, PA

spin labeling strategies

Spin Labeling Strategies

●Continuous ASL (CASL) vs. Pulsed ASL (PASL)

Greater SNR, higher MT effect & SAR, technically challenging

● Pseudo-CASL (pCASL)

Combines advantages of PASL and CASL

● Blood water is used as a natural tracer, with a half-life of blood T1

advantages of high field asl

Advantages of High Field ASL

T1 follows cube root increase with field strength (ω0.3), e.g. blood T1 = 1.2s at 1.5T, 1.5s at 3T, 2s at 7T

Substantial ASL signal gain due to SNR and prolonged T1 at high field (ω1.3).

Wang et al MRM 2002

t2 t2 effect at high field

T2/T2* Effect at High Field

Estimated perfusion signal gain at high field strength with TE=20ms.

Experimental validation, approximately 2 times SNR at 4T vs. 1.5T. CASL is 1.5 SNR vs. PASL.

Wang et al MRM 2002

te dependence of asl

TE Dependence of ASL

St Lawrence & Wang MRM 2005

sample asl image at 7t

Sample ASL Image at 7T

CASL with separate neck labeling coil by NINDS group

Talagala et al ISMRM Perfusion workshop 2007

pseudo continuous asl
Pseudo-continuous ASL
  • 50% SNR gain compared to PASL
  • 30% efficiency gain compared to CASL
  • Expect impressive image quality at 7T

Garcia, de Bazelaire & Alsop, ISMRM 2005

Wu et al. MRM 2007

slide9

Single-shot 3D Perfusion Imaging with GRASE

SPM (p<0.001, n=5) results demonstrating significantly increased perfusion sensitivity in orbitofrontal cortex with GRASE as compared to EPI.

Fernandez-Seara et al. MRM (2005)

slide10

Trade High SNR for High Spatial Resolution at 7T

Increased ASL and BOLD signal change with higher spatial resolution

Pfeufferet al. MRM (2002)

slide11

High Resolution Perfusion Imaging with

pCASL, Background Suppression & Array Coil

Wang et al (2007)

bold fmri

BOLD signal is proportional to the static magnetic field strength (B0) for large vessels (diameter > 8 μm, venules and veins) and to B02 for small vessels (diameter < 8 μm, capillaries) (Ogawa et al 1993)

Change of transverse relaxation rate ∆R*2 in the capillary bed by a power of the field strength varying from 1 to 2 (Turner et al., 1993)

Validation studies by Yang et al 1999, Fera et al 2004, Okada et al 2005.

BOLD fMRI

Okada et al Acad Radio 2005

asl perfusion fmri

ASL Perfusion fMRI

Wang et al MRM 2002

two compartment perfusion model

Two-Compartment Perfusion Model

Blood in capillary and vein is affected by T2/T2* effects.

Solution: shorter TE and post-labeling delay

St Lawrence & Wang MRM 2005

physiological noise at high field

Physiological Noise at High Field

Kruger & Glover MRM 2001

Triantafyllou et al NeuroImage 2005

physiological noise in asl

Physiological Noise in ASL

Wu et al (in preparation)

3d grase asl with background suppression
3D GRASE ASL with Background Suppression

NO SUPPRESSION

  • 10X suppression of temporal fluctuation
  • Benefit at high field

0.7%

WITH SUPPRESSION

8.4%

Fernandez-Seara et al. MRM 2008

slide18

Power Deposition at High Field (SAR)

SAR increases 4 fold at 7T vs. 3TPASL is ok, for CASL/pCASL parameter optimization, separate labeling coil?

Wang et al. Radiology 2005

estimation of venous t2 and oxygenation

Estimation of Venous T2* and Oxygenation

For grey and white matter respectively, CBF = 70.2 ±

9.3 and 41.1 ± 7.6 ml/100g/min, and venous T2*

= 10.8 ± 4.2 and 9.3 ± 4.7 ms. Sensitivity may improve at 7 T

St Lawrence & Wang ISMRM 2005

summary

Summary

Potential large SNR gain at 7T – high spatial resolution

Creative ways to deal with SAR, T2* effects, implementing PASL, CASL & pCASL

Effective approaches to reduce physiological noise

Potentials to measure blood oxygenation, permeability

Utilizing parallel excitation for localized labeling