1. Arterial Spin Labeling at 7T - Double Edged Sword
Jiong-Jiong Wang, PhD
University of Pennsylvania, Philadelphia, PA

2. 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

3. 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

4. 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

5. TE Dependence of ASL
St Lawrence & Wang MRM 2005

6. ASL vs Field Strength - Revisited

7. Sample ASL Image at 7T
CASL with separate neck labeling coil by NINDS group
Talagala et al ISMRM Perfusion workshop 2007

8. 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

9. 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.
Quantitative perfusion MRI with single-shot 3D GRASE are rate-2 GRAPPA in one dimension, yielding 4mm isotropic resolution and whole brain coverage. The images shown are for 50 label/control pairs.
Fernandez-Seara et al. MRM (2005)

10. Trade High SNR for High Spatial Resolution at 7T
Increased ASL and BOLD signal change with higher spatial resolution
Pfeuffer et al. MRM (2002)

11. High Resolution Perfusion Imaging with
pCASL, Background Suppression & Array Coil
High field high resolution (voxel size=2x2x5mm3) pseudo-CASL perfusion images acquired on 3T using an 8-channel head array coil, showing excellent delineation of cortical/subcortical structures.
Wang et al (2007)

12. 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.
Okada et al Acad Radio 2005

13. ASL Perfusion fMRI
Wang et al MRM 2002

14. 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

15. Physiological Noise at High Field
Kruger & Glover MRM 2001
Triantafyllou et al NeuroImage 2005

16. Physiological Noise in ASL
Wu et al (in preparation)

17. 3D GRASE ASL with Background Suppression
NO SUPPRESSION
0.7%
WITH SUPPRESSION
8.4%
10X suppression of temporal fluctuation
Benefit at high field
Fernandez-Seara et al. MRM 2008

18. Power Deposition at High Field (SAR)
SAR increases 4 fold at 7T vs. 3T PASL is ok, for CASL/pCASL parameter optimization, separate labeling coil?
Wang et al. Radiology 2005

19. Power Deposition at High Field (SAR)
B1 G (or RF Gap) can be reduced without affecting efficiency for CASL and pCASL
Maccotta et al. NMR Biomed 1997

20. New Capabilities – Parallel Transmit
Localized labeling ?

21. 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

22. Estimation of Water Permeability using DW ASL
Wang et al JCBFM 2007

23. 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