Diffusion-weighted stimulated echo acquisition mode (DW-STEAM) MR spectroscopy to measure fat unsaturation in regions with low proton-density fat fraction. 
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.  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.  STEAM with TE and mixing time (TM) up to 45 ms exhibited R(2) 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/mm(2), 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.  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.