Robust Fat Suppression for High-Resolution DWI at 5 T Using Slice-Selection Gradient Modulation and Chemical Shift...

Chemical shift encoding (Dixon) can simultaneously remove multiple fat peaks in multi-shot EPI diffusion-weighted imaging (ms-EPI DWI) at 3 T with low sensitivity to B₀ inhomogeneity. However, this method is not applicable at 5 T, since increased fat off-resonance frequencies cause slice position mismatches between different fat peaks. In this work, we propose a two-step strategy combining slice-selection gradient modulation (SSGM) and Dixon to enhance fat suppression for ms-EPI DWI at 5 T.

In the first step, SSGM adjusts the amplitudes of excitation and refocusing slice-selection gradients according to the off-resonance frequencies of methyl/methylene fat peaks, so that these fat slices are excited but not refocused. In the second step, the olefinic fat peak is chemical shift encoded and separated from the diffusion-weighted water images through a joint water/fat separation algorithm with structured low-rank regularization. The two-step strategy was evaluated in the leg, head-and-neck, and prostate.

In vivo experiments demonstrated that the Dixon-only methods cannot simultaneously suppress all fat peaks at 5 T, while this problem was addressed by combining SSGM and Dixon. SSGM showed superior suppression for methyl/methylene fat compared to SPAIR. The following Dixon further removed olefinic fat untouched by SPAIR. Qualitative analysis showed improved overall image quality for all anatomies. Prostate experiments showed that the proposed method is also applicable in reduced FOV acquisitions, high-resolution (1.6-mm isotropic) and high b value imaging (2800 s/mm²).

The proposed two-step strategy improved fat suppression in ms-EPI DWI at 5 T, which can potentially enhance whole-body disease screening and diagnosis.