Easy-to-use B 1 + shims for human brain imaging at 7 T

Abstract

Purpose: $B_1^{+}$ field inhomogeneity is a common problem in high field brain MRI ( $>3$ T). Parallel-transmit methods that adjust the $B_1^{+}$ field channelwise often require valuable scan time. Group-optimized phase shims are presented to increase or attenuate the $B_1^{+}$ field in specific brain regions, omitting personalized calibrations and potentially enabling reduced FOV acquisitions or artifact reduction.

Methods: Channelwise $B_1^{+}$ maps were obtained for seven participants using an 8Tx/32Rx coil and a 7 T MRI scanner. Two regional shim settings ( $B_1^{+}$ shims) were calculated: one to increase the $B_1^{+}$ field in the cerebellum and the other to increase the $B_1^{+}$ field in the occipital lobe while attenuating the $B_1^{+}$ field in the frontal lobe. $B_1^{+}$ maps from five participants outside the design group were used to simulate the $B_1^{+}$ profiles, and seven were scanned to evaluate the implementation of the $B_1^{+}$ shims using $B_1^{+}$ maps, 3D EPI, GRE acquisitions, and a visual fMRI experiment.

Results: Both regional shim settings successfully amplified the $B_1^{+}$ field in the selected ROIs resulting in improved $B_1^{+}$ yield and increased tSNR in the 3D EPI images and fMRI experiments compared to the circularly polarized shim mode. The attenuating $B_1^{+}$ shim decreased $B_1^{+}$ in the frontal ROI, decreasing fold-over artifacts in a reduced FOV, lowering g-factors in accelerated scans with high undersampling factors and resulted in improved BOLD responses in the visual fMRI experiment.

Conclusion: Regional $B_1^{+}$ shim settings remove the need for time-consuming, personalized $B_1^{+}$ measurements and calibrations. The attenuating shim allows for signal reduction within the power limits of the rf-coil, reducing artifacts while improving the $B_1^{+}$ field in selected ROIs.

Keywords: B 1 + $$ {B}_1^{+} $$ shimming; 7 T; calibration‐free; fMRI; high field; phase shimming.