Arterial Spin Labeling at 7T - Double Edged Sword

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Arterial Spin Labeling at 7T- Double Edged Sword

Jiong-Jiong Wang, PhD University of
Pennsylvania, Philadelphia, PA
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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

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Advantages of High Field ASL

T1 follows cube root increase with field strength
(?0.3), 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 (?1.3).
Wang et al MRM 2002



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T2/T2 Effect at High Field

Estimated perfusion signal gain at high field
strength with TE20ms.
Experimental validation, approximately 2 times
SNR at 4T vs. 1.5T. CASL is 1.5 SNR vs. PASL.
Wang et al MRM 2002



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TE Dependence of ASL
St Lawrence Wang MRM 2005
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ASL vs Field Strength - Revisited
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Sample ASL Image at 7T
CASL with separate neck labeling coil by NINDS
group
Talagala et al ISMRM Perfusion workshop 2007
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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
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Single-shot 3D Perfusion Imaging with GRASE

SPM (plt0.001, n5) 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)
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Trade High SNR for High Spatial Resolution at 7T

Increased ASL and BOLD signal change with higher
spatial resolution
Pfeuffer et al. MRM (2002)
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High Resolution Perfusion Imaging with pCASL,
Background Suppression Array Coil

High field high resolution (voxel size2x2x5mm3) 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)
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BOLD fMRI

BOLD signal is proportional to the static
magnetic field strength (B0) for large vessels
(diameter gt 8 µm, venules and veins) and to B02
for small vessels (diameter lt 8 µm, capillaries)
(Ogawa et al 1993) Change of transverse
relaxation rate ?R2 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
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ASL Perfusion fMRI
Wang et al MRM 2002
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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
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Physiological Noise at High Field
Kruger Glover MRM 2001 Triantafyllou et al
NeuroImage 2005
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Physiological Noise in ASL
Wu et al (in preparation)
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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
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SAR increases 4 fold at 7T vs. 3TPASL is ok, for
CASL/pCASL parameter optimization, separate
labeling coil?
Power Deposition at High Field (SAR)
Wang et al. Radiology 2005
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B1 G (or RF Gap) can be reduced without affecting
efficiency for CASL and pCASL
Power Deposition at High Field (SAR)
Maccotta et al. NMR Biomed 1997
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New Capabilities Parallel Transmit

• Localized labeling ?

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