An experimental investigation of labeling efficiency for pseudo-continuous arterial spin labeling

Cerebral blood flow (CBF) is an important physiological metric of neurovascular health that can be measured non-invasively using Arterial Spin Labeling (ASL) MRI. Labeling efficiency (LE) refers to the percentage of the blood spins recruited for the flow estimation and is a crucial parameter for ASL quantification. Traditionally, a labeling efficiency of 0.85 is assumed based on Bloch simulations of spins moving across the labeling plane at laminar, constant physiological velocities. In practice, however, LE is highly dependent on blood velocities, which vary across the cardiac cycle and across the vessel. Here, Bloch simulations for LE estimation were performed using time-varying velocities (TVV). The agreement between ASL and phase contrast (PC) CBF estimates were compared under three conditions: a) an assumed LE of 0.85, b) LE calculated using the average velocity in the vessel, and c) LE calculated using TVV profiles. Using an assumed labeling efficiency, ASL underestimated PC by 15.6±4.4 ml/100g/min, p<0.003. This bias was eliminated by both velocity correction techniques. The use of TVV yielded systemically higher flows than the average velocity correction (−3.6±3.1 ml/100g/min, p<0.276) as well as the lowest bias with respect to PC (−0.9 ± 3.01 ml/100g/min, p=0.777). Velocity correction is an important procedure in ASL quantification, particularly in populations such as children and anemic subjects where blood flow velocity is highly variable.