Microvascular specificity in velocity-selective arterial spin labeling: Effects of the post-labeling delay

Velocity-selective ASL labels arterial blood spins close to the microvasculature if a sufficiently low velocity cutoff (Vcut) is used. Lowering Vcut, however, introduces diffusion and eddy current effects that confound the CBF measurement. A Vcut of 2 cm/s is typical to minimize these effects, but labels blood slightly upstream from the microvasculature; in this scenario, a non-zero post-labeling delay (PLD) allows time for additional microvascular delivery. We evaluate microvascular specificity (using the spatial coefficient of variation of CBF) as a function of PLD. We find that macrovascular specificity increases with PLD up to around 500ms for a typical ASL resolution.