Improving the Robustness of Pseudo-Continuous Arterial Spin Labeling to Off-Resonance and Pulsatile Flow Velocity

Abstract

Purpose To improve pseudo continuous arterial spin labeling (PCASL) robustness to off-resonance and pulsatile blood flow velocity.

Methods The Bloch equations were solved to evaluate the effect of labeling parameters in a pulsatile flow model for a range of off-resonance. Experimental confirmation was achieved in volunteers using linear phase increase between labeling pulses to approximate off-resonance errors. The location of the labeling plane was first assessed on four volunteers, then a range of parameters, including balanced and unbalanced gradients, were explored in five more volunteers at an optimal labeling plane location.

Results Simulations demonstrated that high velocities are vulnerable to off-resonance, that unbalanced PCASL outperforms balanced PCASL, that increased B1 and low average gradient improve the labeling efficiency for high velocity flow, and a low ratio of selective to average gradient improves off-resonance robustness. A good agreement between theory and experiment was observed.

Conclusion The robustness of PCASL can be increased by selecting an unbalanced scheme with a low average gradient (0.5mT/m), a low ratio (7x) of selective to average gradients and the highest feasible B1 (1.8uT). Placing the labeling plane above the carotid bifurcation and below the V3 segment, usually between the second and third vertebrae, produces robust results.