Arterial Spin Labeling at 7TArterial Spin Labeling at 7T- Double Edged Sword- Double Edged Sword

Jiong-Jiong Wang, PhD

University of Pennsylvania, Philadelphia, PA

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

Greater SNR, higher MT effect & SAR, technically challengingGreater SNR, higher MT effect & SAR, technically challenging

● ● Pseudo-CASL (pCASL) Pseudo-CASL (pCASL)

Combines advantages of PASL and CASLCombines advantages of PASL and CASL

● ● Blood water is used as a natural tracer, with a half-life of Blood water is used as a natural tracer, with a half-life of blood T1blood T1

Spin Labeling Strategies Spin Labeling Strategies

Advantages of High Field Advantages of High Field ASLASL

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

Wang et al MRM 2002

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

T2/T2* Effect at High FieldT2/T2* Effect at High Field

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

Wang et al MRM 2002

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

TE Dependence of ASLTE Dependence of ASL

St Lawrence & Wang MRM 2005

ASL vs Field Strength - ASL vs Field Strength - RevisitedRevisited

Sample ASL Image at 7TSample ASL Image at 7T

Talagala et al ISMRM Perfusion workshop 2007

CASL with separate neck labeling coil by NINDS group

Pseudo-continuous ASLPseudo-continuous ASL

50% SNR gain compared to PASL50% SNR gain compared to PASL 30% efficiency gain compared to CASL30% efficiency gain compared to CASL Expect impressive image quality at 7TExpect impressive image quality at 7T

Garcia, de Bazelaire & Alsop, Garcia, de Bazelaire & Alsop, ISMRMISMRM 2005 2005Wu et al. MRM 2007Wu et al. MRM 2007

Single-shot 3D Perfusion Imaging with GRASE

Quantitative perfusion MRI with single-shot 3D Quantitative perfusion MRI with single-shot 3D GRASE are rate-2 GRAPPA in one dimension, GRASE are rate-2 GRAPPA in one dimension, yielding 4mm isotropic resolution and whole brain yielding 4mm isotropic resolution and whole brain coverage. The images shown are for 50 coverage. The images shown are for 50 label/control pairs.label/control pairs.

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)

Trade High SNR for High Spatial Resolution at 7T

Increased ASL and BOLD signal change with higher spatial resolution

Pfeuffer et al. MRM (2002)

High Resolution Perfusion Imaging with

pCASL, Background Suppression & Array Coil

High field high resolution (voxel size=2x2x5mmHigh field high resolution (voxel size=2x2x5mm33) pseudo-CASL ) pseudo-CASL perfusion images acquired on 3T using an 8-channel head array coil, perfusion images acquired on 3T using an 8-channel head array coil, showing excellent delineation of cortical/subcortical structures. showing excellent delineation of cortical/subcortical structures.

Wang et al (2007)

BOLD fMRIBOLD 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.

Okada et al Acad Radio 2005

ASL Perfusion fMRIASL Perfusion fMRI

Wang et al MRM 2002

Two-Compartment Perfusion Two-Compartment Perfusion ModelModel

St Lawrence & Wang MRM 2005

Blood in capillary and vein is affected by T2/T2* effects. Solution: shorter TE and post-labeling delay

artery veintissue

1 2 3

artery veintissue

1 2 3

Physiological Noise at High FieldPhysiological Noise at High Field

Kruger & Glover MRM 2001

Triantafyllou et al NeuroImage 2005

Physiological Noise in ASLPhysiological Noise in ASL

Wu et al (in preparation)

3D GRASE ASL with Background 3D GRASE ASL with Background SuppressionSuppression

0.7%0.7%

8.4%8.4%

WITH SUPPRESSION

NO SUPPRESSION

10X suppression of temporal fluctuation10X suppression of temporal fluctuation Benefit at high fieldBenefit at high field

Fernandez-Seara et al. MRM 2008

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

Power Deposition at High Field Power Deposition at High Field (SAR)(SAR)

Wang et al. Radiology 2005

B1 G (or RF Gap) can be reduced B1 G (or RF Gap) can be reduced without affecting efficiency for CASL without affecting efficiency for CASL and pCASLand pCASL

Power Deposition at High Field Power Deposition at High Field (SAR)(SAR)

Maccotta et al. NMR Biomed 1997

New Capabilities – Parallel New Capabilities – Parallel Transmit Transmit

Localized labeling ?Localized labeling ?

Estimation of Venous T2* and Estimation of Venous T2* and OxygenationOxygenation

St Lawrence & Wang ISMRM 2005

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

Estimation of Water Estimation of Water Permeability using DW ASLPermeability using DW ASL

Wang et al JCBFM 2007

SummarySummary

Potential large SNR gain at 7T – high spatial Potential large SNR gain at 7T – high spatial resolutionresolution

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

Effective approaches to reduce physiological Effective approaches to reduce physiological noisenoise

4.4. Potentials to measure blood oxygenation, Potentials to measure blood oxygenation, permeability permeability

5.5. Utilizing parallel excitation for localized Utilizing parallel excitation for localized labelinglabeling