Abstract
PURPOSE: B1+ field inhomogeneity is a common problem in high field brain MRI ( > 3 T). Parallel-transmit methods that adjust the B1+ field channelwise often require valuable scan time. Group-optimized phase shims are presented to increase or attenuate the B1+ field in specific brain regions, omitting personalized calibrations and potentially enabling reduced FOV acquisitions or artifact reduction. METHODS: Channelwise B1+ maps were obtained for seven participants using an 8Tx/32Rx coil and a 7 T MRI scanner. Two regional shim settings ( B1+ shims) were calculated: one to increase the B1+ field in the cerebellum and the other to increase the B1+ field in the occipital lobe while attenuating the B1+ field in the frontal lobe. B1+ maps from five participants outside the design group were used to simulate the B1+ profiles, and seven were scanned to evaluate the implementation of the B1+ shims using B1+ maps, 3D EPI, GRE acquisitions, and a visual fMRI experiment. RESULTS: Both regional shim settings successfully amplified the B1+ field in the selected ROIs resulting in improved B1+ yield and increased tSNR in the 3D EPI images and fMRI experiments compared to the circularly polarized shim mode. The attenuating B1+ shim decreased B1+ 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 B1+ shim settings remove the need for time-consuming, personalized B1+ measurements and calibrations. The attenuating shim allows for signal reduction within the power limits of the rf-coil, reducing artifacts while improving the B1+ field in selected ROIs.