Diffusion-weighted stimulated echo acquisition mode (DW-STEAM) MR spectroscopy to measure fat unsaturation in regions with low proton-density fat fraction

Purpose To propose and optimize diffusion-weighted stimulated echo acquisition mode (DW-STEAM) for measuring fat unsaturation in the presence of a strong water signal by suppressing the water signal based on a shorter T₂ and higher diffusivity of water relative to fat. Methods A parameter study for point-resolved spectroscopy (PRESS) and STEAM using oil phantoms was performed and correlated with gas chromatography (GC). Simulations of muscle tissue signal behavior using DW-STEAM and long-echo time (TE) PRESS and a parameter optimization for DW-STEAM were conducted. DW-STEAM and long-TE PRESS were applied in the gastrocnemius muscles of nine healthy subjects. Results STEAM with TE and mixing time (TM) up to 45 ms exhibited R² correlations above 0.98 with GC and little T₂-weighting and J-modulation for the quantified olefinic/methylene peak ratio. The optimal parameters for muscle tissue using DW-STEAM were b-value = 1800 s/mm², TE = 33 ms, TM = 30 ms, and repetition time = 2300 ms. In vivo measured mean olefinic signal-to-noise ratios were 72 and 40, mean apparent olefinic water fractions were 0.19 and 0.11 for DW-STEAM and long-TE PRESS, respectively. Conclusion Optimized DW-STEAM MR spectroscopy is superior to long-TE PRESS for measuring fat unsaturation, if a strong water peak prevents the olefinic fat signal's quantification at shorter TEs and water's tissue specific ADC is substantially higher than fat.