Intravascular effect in velocity-selective arterial spin labeling: the choice of inflow time and cutoff velocity

Velocity-selective arterial spin labeling (VS-ASL) tags spins on a basis of flow velocity, instead of spatial distribution that has been commonly adopted in conventional ASL techniques. VS-ASL can potentially generate tags that are very close to the imaging plane and whereby avoid the error source of transit delay (deltat) variation independent of inflow time (TI). In practice, however, TI of VS-ASL should still be chosen with caution with respect to intravascular signal and cutoff velocity (V(c)). The presented study takes advantage of multiple TI and V(c) to systematically investigate the intravascular effect. Results demonstrate the presence of significant signal from large vessels in VS-ASL images for V(c) down to 4 cm/s. For perfusion measurement in human brain, low V(c) (<4 cm/s) is recommended. With V(c) = 2 cm/s, quantitative cerebral blood flow is 72.8 ml/100 ml/min, which is in agreement with the reported range using conventional ASL methods. In field strength of 3 T, numerical simulation shows that optimal signal-to-noise ratio efficiency can be achieved with TR/TI = 2092 ms/1664 ms for single slice and 4493 ms/1404 ms for slab imaging.