Removal of Olefinic Fat Signal in Body Diffusion-Weighted EPI Using a Dixon Method

Introduction Body fat produces a complex set of peaks in the MR signal. Diffusion-Weighted (DW) body MR imaging is typically performed using EPI with chemical shift-based fat suppression, e.g., spatial-spectral (SS) excitation pulses. SS pulses effectively suppress the main fat peak (methylene protons, ~1.4 ppm) as well as other secondary peaks with similar chemical shifts. However, they fail to suppress the signal due to the olefinic protons (~5.4 ppm), which is spectrally close to the water peak (see Fig. 1). In the presence of DW gradients, the water signal is greatly attenuated, causing the slow-diffusing olefinic fat signal to become significant. Additionally, the chemical shift artifact (CSA) in EPI results in the subcutaneous fat signal often overlapping the region of interest (i.e., muscle), Fig. 1: In vivo spectrum, including water and several fat peaks.