Optimization of pseudo-continuous arterial spin labeling at 7T with parallel transmission B1 shimming

Abstract

Purpose: To optimize pseudo-continuous arterial spin labeling (pCASL) for 7 T, and to further improve the labeling efficiency with parallel RF transmission transmit B₁ ( $B_1^{+}$ ) shimming.

Methods: pCASL parameters were optimized based on $B_1^{+}/B_0$ field distributions at 7 T with simulation. To increase labeling efficiency, the $B_1^{+}$ amplitude at inflowing arteries was increased with parallel RF transmission $B_1^{+}$ shimming. The "indv-shim" with shimming weights calculated for each individual subject, and the "univ-shim" with universal weights calculated on a group of 12 subjects, were compared with circular polarized (CP) shim. The optimized pCASL sequences with three $B_1^{+}$ shimming modes (indv-shim, univ-shim, and CP-shim) were evaluated in 6 subjects who underwent two repeated scans 24 hours apart, along with a pulsed ASL sequence. Quantitative metrics including mean $B_1^{+}$ amplitude, perfusion, and intraclass correlation coefficient were calculated. The optimized 7T pCASL was compared with standard 3T pCASL on 5 subjects, using spatial SNR and temporal SNR.

Results: The optimal pCASL parameter set (RF duration/gap = 300/250 us, $G_{\mathrm{ave}}=0.6\text{mT}/m,gRatio=10$ ) achieved robust perfusion measurement in the presence of $B_1^{+}/B_0$ inhomogeneities. Both indv-shim and univ-shim significantly increased $B_1^{+}$ amplitude compared with CP-shim in simulation and in vivo experiment (P < .01). Compared with CP-shim, perfusion signal was increased by 9.5% with indv-shim (P < .05) and by 5.3% with univ-shim (P = .35). All three pCASL sequences achieved fair to good repeatability (intraclass correlation coefficient ≥ 0.5). Compared with 3T pCASL, the optimized 7T pCASL achieved 78.3% higher spatial SNR and 200% higher temporal SNR.

Conclusion: The optimized pCASL achieved robust perfusion imaging at 7 T, while both indv-shim and univ-shim further increased labeling efficiency.

Keywords: ${\mathrm{B}}_1^{+}$ shimming; arterial spin labeling; parallel RF transmission; perfusion; ultrahigh field; universal pulse.