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

PURPOSE: B 1 + $$ {B}_1^{+} $$ field inhomogeneity is a common problem in high field brain MRI ( > 3 $$ >3 $$  T). Parallel-transmit methods that adjust the B 1 + $$ {B}_1^{+} $$ field channelwise often require valuable scan time. Group-optimized phase shims are presented to increase or attenuate the B 1 + $$ {B}_1^{+} $$ field in specific brain regions, omitting personalized calibrations and potentially enabling reduced FOV acquisitions or artifact reduction.

METHODS: Channelwise B 1 + $$ {B}_1^{+} $$ maps were obtained for seven participants using an 8Tx/32Rx coil and a 7 T MRI scanner. Two regional shim settings ( B 1 + $$ {B}_1^{+} $$ shims) were calculated: one to increase the B 1 + $$ {B}_1^{+} $$ field in the cerebellum and the other to increase the B 1 + $$ {B}_1^{+} $$ field in the occipital lobe while attenuating the B 1 + $$ {B}_1^{+} $$ field in the frontal lobe. B 1 + $$ {B}_1^{+} $$ maps from five participants outside the design group were used to simulate the B 1 + $$ {B}_1^{+} $$ profiles, and seven were scanned to evaluate the implementation of the B 1 + $$ {B}_1^{+} $$ shims using B 1 + $$ {B}_1^{+} $$ maps, 3D EPI, GRE acquisitions, and a visual fMRI experiment.

RESULTS: Both regional shim settings successfully amplified the B 1 + $$ {B}_1^{+} $$ field in the selected ROIs resulting in improved B 1 + $$ {B}_1^{+} $$ yield and increased tSNR in the 3D EPI images and fMRI experiments compared to the circularly polarized shim mode. The attenuating B 1 + $$ {B}_1^{+} $$ shim decreased B 1 + $$ {B}_1^{+} $$ 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 + $$ {B}_1^{+} $$ shim settings remove the need for time-consuming, personalized B 1 + $$ {B}_1^{+} $$ 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 + $$ {B}_1^{+} $$ field in selected ROIs.