Improving PCASL at ultra-high field using a VERSE-guided parallel transmission strategy

Purpose: To improve the labeling efficiency of pseudo-continuous arterial spin labeling (PCASL) at 7T using parallel transmission (pTx).

Methods: Five healthy subjects were scanned on an 8-channel-transmit 7T human MRI scanner. Time-of-flight (TOF) angiography was acquired to identify regions of interest (ROIs) around the 4 major feeding arteries to the brain, and $B_1^{+}$ and B₀ maps were acquired in the labeling plane for tagging pulse design. Complex weights of the labeling pulses for each of the 8 transmit channels were calculated to produce a homogenous radiofrequency (RF) -shimmed labeling across the ROIs. Variable-Rate Selective Excitation (VERSE) pulses were also implemented as a part of the labeling pulse train. Whole-brain perfusion-weighted images were acquired under conditions of RF shimming, VERSE with RF shimming, and standard circularly polarized (CP) mode. The same subjects were scanned on a 3T scanner for comparison.

Results: In simulation, VERSE with RF shimming improved the flip-angles across the ROIs in the labeling plane by 90% compared with CP mode. VERSE with RF shimming improved the temporal signal-to-noise ratio by 375% compared with CP mode, but did not outperform a matched 3T sequence with a matched flip-angle.

Conclusion: We have demonstrated improved PCASL tagging at 7T using VERSE with RF shimming on a commercial head coil under conservative SAR limits at 7T. However, improvements of 7T over 3T may require strategies with less conservative SAR restrictions.