Development and application of Arterial Spin Labeling MRI in human brain disorders

Pseudo-Continuous Arterial Spin Labeling (pCASL) MRI is a new ASL technique that has the potential of combining advantages of Continuous-ASL and Pulsed-ASL. However, unlike CASL, the labeling process of pCASL is not strictly an adiabatic inversion and the efficiency of labeling may be subject-specific.

We performed three experiments to study the labeling efficiency in pCASL MRI:

  • The optimal labeling position was determined empirically to be 84 mm below Anterior-Commissure Posterior-Commissure line in order to achieve the highest sensitivity.  
  • An experimental method was developed to utilize phase-contrast MRI as a normalization factor and to estimate the labeling efficiency in vivo, which was founded to be 0.86±0.06 (n=10, mean±SD).  
  • We compared the labeling efficiency of pCASL MRI under normocapnic and hypercapnic (inhalation of 5 percent CO2) conditions, and showed that a higher flow velocity in the feeding arteries resulted in a reduction in the labeling efficiency. 

In summary, our results suggest that labeling efficiency is a critical parameter in pCASL MRI not only in terms of achieving highest sensitivity but also in quantification of absolute CBF in ml/min/100g. We propose that the labeling efficiency should be estimated using phase-contrast MRI on a subject-specific and physiologic-condition-specific basis.

Brain scans
Intensity of CBF-weighted signal as a function of labeling position.
Brain scans
Top: Phase-contrast (PC) MRI of Internal Carotid Arteries (ICA) and Vertebral Arteries (VA). (a) Location of the slice of PC MRI on a mid-sagittal image. (b) Location of the PC MRI on the angiogram. (c) Raw image of PC MRI. (d) Magnitude image of PC MRI. The insets show the left and right ICAs and VAs as well as the manually drawn ROIs. (e) Velocity map from the PC MRI. 
Bottom: Average CBF maps during normocapnia and hypercapnia as well their differences.