Hybrid B1+ -shimming and gradient adaptions for improved pseudo-continuous arterial spin labeling at 7 Tesla

Abstract

Purpose: To improve pseudo-continuous arterial spin labeling (pcASL) at 7T by exploiting a hybrid homogeneity- and efficiency-optimized $B_1^{+}$ -shim with adapted gradient strength as well as background suppression.

Methods: The following three experiments were performed at 7T, each employing five volunteers: (1) A hybrid (ie, homogeneity-efficiency optimized) $B_1^{+}$ -shim was introduced and evaluated for variable-rate selective excitation pcASL labeling. Therefore, $B_1^{+}$ -maps in the V3 segment and time-of-flight images were acquired to identify the feeding arteries. For validation, a gradient-echo sequence was applied in circular polarized (CP) mode and with the hybrid $B_1^{+}$ -shim. Additionally, the gray matter (temporal) signal-to-noise ratio (tSNR) in pcASL perfusion images was evaluated. (2) Bloch simulations for the pcASL labeling were conducted and validated experimentally, with a focus on the slice-selective gradients. (3) Background suppression was added to the $B_1^{+}$ -shimmed, gradient-adapted 7T sequence and this was then compared to a matched sequence at 3T.

Results: The $B_1^{+}$ -shim improved the signal within the labeling plane (23.6%) and the SNR/tSNR increased (+11%/+11%) compared to its value in CP mode; however, the increase was not significant. In accordance with the simulations, the adapted gradients increased the tSNR (35%) and SNR (45%) significantly. Background suppression further improved the perfusion images at 7T, and this protocol performed as well as a resolution-matched protocol at 3T.

Conclusion: The combination of the proposed hybrid $B_1^{+}$ -phase-shim with the adapted slice-selective gradients and background suppression shows great potential for improved pcASL labeling under suboptimal $B_1^{+}$ conditions at 7T.

Keywords: 7 Tesla; B1 shimming; VERSE; pTx; pcASL; ultra-high field strengths.