Human Brain High-Resolution Diffusion MRI With Optimized, Slice-By-Slice, Zeroth and First Order B 0 Shimming in...

To propose a brain tissue-selective, optimized slice-by-slice $B_0$ shimming for high-resolution brain diffusion MRI.

We incorporated actual gradient fields of the $X$ , $Y$ , and $Z$ 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 $B_0$ shimming to minimize $B_0$ field inhomogeneity (i.e., $\Delta B_0$ ) 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.

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 $T_2$ FSE hypo-intensity region by applying dynamic shimming. Furthermore, in phantom, the root-mean-square of $\Delta B_0$ in areas with high gradient nonlinearity was reduced by 7 Hz when incorporating the actual gradient field in dynamic shimming.

Diffusion MRI with brain tissue-selective, dynamic slice-by-slice $B_0$ effectively improves diffusivity characterization in high-resolution images.