Optimizing pseudo-continuous ASL at 7 Tesla using dynamic low order-shim update

Motivation: The labeling efficiency of pseudo-continuous ASL (pCASL) is reduced in the presence of large B0 off-resonance, which makes pCASL challenging at ultra-high fields due to susceptibility-induced off-resonance. Goal(s): We aim to significantly reduce the labeling efficiency dependence on B0 off-resonance of pCASL to achieve highest allowed labeling efficiency at a given labeling flip angle. Approach: 0th- (frequency) and 1st-order (x,y,z) shim components were used dynamically in the duration of labeling to mitigate B0 off-resonance without affecting imaging readout. Results: Improved labeling efficiency was achieved, which enabled higher-resolution perfusion imaging using pCASL. Selective labeling of blood was also demonstrated possible using our approach. Impact: Our approach provides a simple method to substantially remove the labeling-efficiency dependence on B0 off-resonance for pCASL at 7T. This enables high-quality ASL-based perfusion imaging at ultra-high fields for study of brain function, physiology, and pathology.