Estimation of labeling efficiency in pseudocontinuous arterial spin labeling

Abstract

Pseudocontinuous arterial spin labeling MRI is a new arterial spin labeling technique that has the potential of combining advantages of continuous arterial spin labeling and pulsed arterial spin labeling. However, unlike continuous arterial spin labeling, the labeling process of pseudocontinuous arterial spin labeling is not strictly an adiabatic inversion and the efficiency of labeling may be subject specific. Here, three experiments were performed to study the labeling efficiency in pseudocontinuous arterial spin labeling MRI. First, the optimal labeling position was determined empirically to be approximately 84 mm below the anterior commissure-posterior commissure line in order to achieve the highest sensitivity. Second, an experimental method was developed to utilize phase-contrast velocity 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 +/- standard deviation). Third, we compared the labeling efficiency of pseudocontinuous arterial spin labeling MRI under normocapnic and hypercapnic (inhalation of 5% CO(2)) 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 pseudocontinuous arterial spin labeling MRI not only in terms of achieving highest sensitivity but also in quantification of absolute cerebral blood flow in milliliters per minute per 100 g. We propose that the labeling efficiency should be estimated using phase-contrast velocity MRI on a subject-specific basis.

Fig. 1

Intensity of CBF-weighted signal as a function of labeling position. PCASL MRI was performed with the labeling plane positioned at 7 different locations (bottom panel). The locations are in reference to the AC-PC line. The top panel shows average CBF-weighted images at each location. Image intensity change as a function of labeling position can be visually observed. Error bars indicate standard errors of mean.

Fig. 2

Phase-contrast (PC) velocity MRI of Internal Carotid Arteries (ICA) and Vertebral Arteries (VA). (a) Location of the slice of PC velocity MRI on a mid-sagittal image. (b) Location of the PC velocity MRI on the angiogram. (c) Raw image of PC velocity MRI. (d) Magnitude image of PC velocity MRI. The insets show the left and right ICAs and VAs as well as the manually drawn ROIs. (e) Velocity map from the PC velocity MRI. Positive value indicates inflow blood, often corresponding to arteries, whereas negative value indicates outflow blood, typically from veins.

Fig. 3

Representative CBF maps during normocapnia and hypercapnia as well their differences. The data have been spatially normalized to the MNI brain template. The quantification of CBF used Eq. [2] and the experimentally estimated labeling efficiency. CBF increases can be seen in the entire brain.

Fig. 4

Experimental and simulation data of pCASL labeling efficiency as a function of flow velocity. The experimental data (filled dots) were from all subjects in Studies 2 and 3 (26 measurements). The solid line is the linear fitting of the experimental data. The simulation (dashed curve) used sequence parameters identical to those used in the experiments: RF duration=0.5 ms, pause between RF pulses=0.5 ms, labeling pulse flip angle=18°. The spins were assumed to be on-resonance and flowing at a constant velocity. The experimental data showed a significant correlation between labeling efficiency and flow velocity (r=−0.46, p=0.02).