Human Brain High‐Resolution Diffusion MRI With Optimized, Slice‐By‐Slice, Zeroth and First Order B0 B₀ Shimming in Head‐Only High‐Gradient MRI Systems

ABSTRACT Purpose To propose a brain tissue‐selective, optimized slice‐by‐slice shimming for high‐resolution brain diffusion MRI. Methods We incorporated actual gradient fields of the , , and imaging gradient coils, which are already designed for fast switching and accessible to any MRI system, in the calculation of the shimming coefficients in dynamic slice‐by‐slice shimming to minimize field inhomogeneity (i.e., ) in deep‐learning segmented brain tissues. Diffusion MRI with oscillating gradient spin echo (OGSE) at 55 Hz and pulsed gradient spin echo (PGSE) (approximated at 0 Hz) were obtained in phantoms and healthy volunteers using a head‐only high‐gradient 3T MRI system. In each diffusion MRI acquisition, standard static volumetric shimming and the proposed shimming were applied separately, and mean/axial/radial diffusivities (MD/AD/RD) and fractional anisotropy (FA) were estimated. Results Compared to static shimming, dynamic shimming reduced the root‐mean‐square of voxel displacement of each slice by a maximum of 5–10 voxels in single‐shot EPI acquisition at 1–2 mm in‐plane resolution in the phantom, and a maximum of 3 voxels in human brains. Improved accuracy of MD/AD/RD/FA in the frontal lobe, brainstem, and cerebellum was observed by applying dynamic shimming and/or two‐shot EPI acquisition. MD (55 Hz)–MD (0 Hz) showed higher values in FSE hypo‐intensity region by applying dynamic shimming. Furthermore, in phantom, the root‐mean‐square of in areas with high gradient nonlinearity was reduced by 7 Hz when incorporating the actual gradient field in dynamic shimming. Conclusion Diffusion MRI with brain tissue‐selective, dynamic slice‐by‐slice effectively improves diffusivity characterization in high‐resolution images.