Understanding and optimizing the amplitude modulated control for multiple‐slice continuous arterial spin labeling

Multiple-slice perfusion imaging by continuous arterial spin labeling (CASL) is made possible by amplitude modulation (AM) of the labeling RF pulse, but perfusion sensitivity is reduced relative to the single-slice technique. A computer model of the Bloch equations for velocity driven adiabatic fast passage was developed to elucidate the compromised sensitivity to perfusion of the AM control technique for CASL. Calculations were performed over ranges of RF pulse amplitude, B-1; magnetic field gradient, G; phase, phi, and frequency, f, of the modulation function; velocity, v, and relaxation times, T-1 and T-2, of blood. It was found that unless f > 2 pi B-1, phi determines the performance of the AM control; excessively high B-1 or v reduces the efficiency of the AM control; and T, relaxation dominates if f is too great. In vivo, in rat brain (n = 5) at 2.35 T, the sensitivity of the AM technique to perfusion was 70% of the sensitivity of single-slice CASL.