Dynamic B 0 field shimming for improving pseudo-continuous arterial spin labeling at 7 Tesla
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Purpose
B 0 inhomogeneity within the brain-feeding arteries is a major issue for pseudo-continuous arterial spin labeling (PCASL) at 7T because it reduces labeling efficiency and leads to a loss of perfusion signal. This study aimed to develop a vessel-specific dynamic B 0 field shimming method for 7T PCASL to enhance labeling efficiency by correcting off-resonance in the arteries within the labeling region.
Methods
We implemented a PCASL sequence with dynamic B₀ shimming at 7T that compensates for B 0 field offsets at 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 shimming parameters. We evaluated both 2D and 3D variants of our method, comparing their performance against established global frequency offset and optimal-encoding-scheme (OES)-based corrections. Cerebral blood flow (CBF) maps were quantified before and after corrections. CBF values from different methods in the whole brain, white matter, and grey matter regions were compared.
Results
All off-resonance correction methods significantly enhanced perfusion signals across the brain. The proposed vessel-specific dynamic B₀ shimming method improved labeling efficiency while maintaining optimal static shimming in the imaging region. Perfusion-weighted images demonstrated the superiority of 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 B 0 shimming method offers a significant advancement in PCASL robustness and effectiveness, enabling full utilization of 7T ASL’s high sensitivity and spatial resolution.
