Dynamic B₀ field shimming for improving pseudo-continuous arterial spin labeling at 7 T

Purpose: B₀ field inhomogeneity within the brain-feeding arteries is a major issue for pseudo-continuous arterial spin labeling (PCASL) at 7 T because it reduces the labeling efficiency and leads to a loss of perfusion signal. This study aimed to develop a vessel-specific dynamic B₀ field shimming method for 7 T PCASL to improve the labeling efficiency by correcting off-resonance within the arteries in the labeling region.

Methods: We implemented a PCASL sequence with dynamic B₀ shimming at 7 T that compensates for B₀ field offsets in the brain-feeding arteries by updating linear shimming terms and adding a phase increment to the PCASL RF pulses. Rapidly acquired vessel-specific B₀ field maps were used to calculate dynamic B₀ shimming parameters. We evaluated both 2D and 3D variants of our method, comparing their performance against the established global frequency offset and optimal encoding scheme-based corrections. Cerebral blood flow (CBF) maps were quantified before and after corrections, and CBF values from different methods were compared across the whole brain, white matter, and gray matter regions.

Results: All off-resonance correction methods significantly recovered perfusion signals across the brain. The proposed vessel-specific dynamic B₀ shimming method improved the labeling efficiency while maintaining optimal static shimming in the imaging region. Perfusion-weighted images demonstrated the superiority of the 3D dynamic B₀ shimming method compared to global or 2D-based correction approaches. CBF analysis revealed that 3D dynamic B₀ shimming significantly increased CBF values relative to the other methods.

Conclusion: Our proposed dynamic B₀ shimming method offers a significant advancement in PCASL robustness and effectiveness, enabling full utilization of 7 T ASL high sensitivity and spatial resolution.