Whole-Brain Diffusion MRI at 7T With Universal Multiband GRAPE pTx Pulses

To improve signal strength and homogeneity of whole-brain diffusion-weighted imaging (DWI) at 7T.

Universal multiband excitation and refocusing pulses were optimized with the gradient ascent pulse engineering (GRAPE) algorithm across a database of $B_1^{+}$ and $B_0$ maps obtained from 20 subjects. Slice-selective multiband pulses were optimized in a two-step optimization for transverse and sagittal slice orientations. The optimized pTx pulses were compared to standard circular polarized (CP) pulses in five subjects using a multiband diffusion-weighted EPI sequence at 1.5 mm isotropic resolution and in one subject at 1.05 mm resolution. The evaluation included Bloch simulations, flip angle mapping, SNR analysis, and diffusion model fitting (DTI and NODDI).

Both GRAPE pulse sets with transverse and sagittal slice orientation improved signal homogeneity and SNR compared to CP pulses across all subjects and head sizes, with particularly high gains in the cerebellum. Sagittal GRAPE pulses yielded the highest mean signal and lowest standard deviation across the brain, while transverse GRAPE pulses provided better cerebellar signal at the cost of signal loss in off-resonant frontal and temporal regions of large heads. The improved signal translated into more reliable DTI and NODDI fitting, especially in the cerebellum.

Universal multiband GRAPE pTx pulses substantially improve signal homogeneity and SNR in whole-brain DWI at 7T, enabling more reliable diffusion microstructure mapping, particularly in the cerebellum.