Intravascular effect in velocity-selective arterial spin labeling

Arterial spin labeling (ASL) has been developed into a useful tool for measuring local tissue perfusion with magnetic resonance imaging (MRI). Velocity-selective ASL (VS-ASL) tags spins on a basis of flow velocity, instead of the spatial distribution that is commonly used by conventional ASL techniques. Using a 90deg-180deg-90deg 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 practice, however, TI of VS-ASL should still be chosen with caution with respect to intravascular signal and cutoff velocity (Vc). With higher Vc, the maximum of VS-ASL signal was observed at shorter TI, which is due to intravascular signal in large vessels. For perfusion measurement in human brain, small Vc (=<2cm/s) is recommended. In contrast to the previous studies that suggested CSF suppression for Vc < 5cm/s, this study used lower amplitude and longer separation of diffusion gradients in the VS pulse train, which reduced b-value and eliminated the necessity of CSF suppression for Vc down to 2 cm/s