A theoretical and experimental investigation of the tagging efficiency of pseudocontinuous arterial spin labeling

TLDR

Arterial spin labeling noninvasively measures blood flow by magnetically tagging arterial protons, and pseudocontinuous ASL uses a train of discrete RF pulses to emulate continuous tagging when true continuous RF transmit is unavailable. The study implemented two versions of pCASL—balanced and unbalanced gradient waveforms in tag and control scans—across transmit/receive coils and array receivers. pCASL achieves tagging by applying a discrete RF pulse train with balanced or unbalanced gradients in the tag and control scans, delivered through transmit/receive coils and array receivers. Experimental data show a 50 % ± 4 % SNR increase over PASL and 80 % tagging efficiency versus 68 % for AM CASL, with simulations predicting an optimal 85 % efficiency for 10–60 cm s⁻¹; tagging efficiency depends on resonance offset and flip angle, and pCASL balances PASL’s lower hardware demand and higher tagging efficiency with CASL’s longer bolus and higher SNR. Published in Magn Reson Med.

Abstract

Abstract Arterial spin labeling (ASL) is capable of noninvasively measuring blood flow by magnetically tagging the protons in arterial blood, which has been conventionally achieved using instantaneous (PASL) or continuous (CASL) RF pulses. As an intermediate method, pseudocontinuous ASL (pCASL) utilizes a train of discrete RF pulses to mimic continuous tagging that is often unavailable on imagers due to the requirement of continuous RF transmit capabilities. In the present study, we implemented two versions of pCASL ( balanced and unbalanced gradient waveforms in tag and control scans) for both transmit/receive coils and array receivers. Experimental data show a 50% ± 4% increase of signal‐to‐noise ratio (SNR) compared with PASL and a higher tagging efficiency than amplitude‐modulated (AM) CASL (80% vs. 68%). Computer simulations predict an optimal tagging efficiency of 85% for flow velocities from 10 to 60 cm/s. It is theoretically and experimentally demonstrated that the tagging efficiency of pCASL is dependent upon the resonance offset and flip angle of the RF pulse train. We conclude that pCASL has the potential of combining the merits of PASL, including less hardware demand and higher tagging efficiency, and CASL, which includes a longer tagging bolus and thus higher SNR. These improvements provide a better balance between tagging efficiency and SNR. Magn Reson Med 58:1020–1027, 2007. © 2007 Wiley‐Liss, Inc.