Feasibility of velocity selective arterial spin labeling in functional MRI.

Arterial spin labeling (ASL) magnetically inverts or saturates the spins in arterial blood and uses them as endogenous tracers. Conventionally, the tagging band is upstream or nonselective to the target slices. In the brain, ASL-based functional magnetic resonance imaging (fMRI) has been shown to detect activation better localized in gray matter than blood oxygenation level dependent contrast. More recently, velocity selective-ASL (VS-ASL) was proposed to tag spins according to their flow velocity. One desirable characteristic of VS-ASL is its capability to generate tags sufficiently close to the target slices and thereby circumvent the complication of non-zero transit delay. In this study, we investigate the feasibility of VS-ASL in fMRI by comparing it with a conventional ASL method (PICORE). The results from the visual cortex of healthy volunteers show that VS-ASL and PICORE have comparable spatial specificity in detecting the flow change induced by neuronal activity. Velocity selective-arterial spin labeling can further distinguish the contribution from different flow directions but spurious elevation of fractional signal change may occur when the VS tagging is applied off the direction of blood supply. The flow reaches the vicinity of perfusion at a cutoff velocity (Vc) of 2 cm/sec whereas the activation exclusively detected by Vc=4 cm/sec implies the arteriolar response to the neuronal activity and a respondent vessel diameter up to 240 microm. Velocity selective imaging can remove intravascular signal from the vessels where the flow velocity is above Vc.