Easy-to-use shims for human brain imaging at 7 T.

PURPOSE

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

METHODS

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

RESULTS

Both regional shim settings successfully amplified the field in the selected ROIs resulting in improved yield and increased tSNR in the 3D EPI images and fMRI experiments compared to the circularly polarized shim mode. The attenuating shim decreased 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 shim settings remove the need for time-consuming, personalized measurements and calibrations. The attenuating shim allows for signal reduction within the power limits of the rf-coil, reducing artifacts while improving the field in selected ROIs.