The effects of water diffusion and laminar flow on velocity-selective arterial spin labeling

Arterial spin labeling (ASL) has been developed as a useful tool for measuring local tissue perfusion with magnetic resonance imaging (MRI). In contrast to conventional ASL that spatially labels spins, velocity-selective ASL (VS-ASL) tags spins on a basis of flow velocity. Using a 90°-180°-90° radiofrequency pulse train in combination with flow-sensitive gradients, VS-ASL can potentially generate tags that are very close to the imaging plane and whereby avoid the main error source of conventional ASL techniques coming from T1 relaxation during inflow time (TI). In this study, computer simulation was used to evaluate the diffusion effect upon the tag distribution during the VS process. The change of tag amount caused by water diffusion was found to be less than 5%. Considerations of the velocity threshold (Vcut) were addressed as well. Large Vcut incurs longer delay for the delivery of tags, compromising the advantage of VS-ASL. A Vcut below 5 cm/s is recommended on the premise of proper CSF suppression.