(Sunday, 7/14/2019) 2:00 PM - 3:00 PM
Room: 221CD
Purpose: Measuring intramyocellular and extramyocellular lipid (IMCL and EMCL) concentrations in skeletal muscle using MR spectroscopy (MRS) is confounded by susceptibility induced chemical shift of the lipid peaks caused by variations muscle fiber orientation with the main magnetic field Bâ‚€, and the presence of fascial fat streaks. We evaluate the impact of fiber orientation within same muscle group (soleus) for quantifying concentrations of IMCL and EMCL.
Methods: Patient population: 21 subjects who provided written informed consent (11 male, 71.2±3.6yrs) were imaged at 3.0T. MR acquisition: Aided by muscle fiber tracks determined by DTI (16 directions, b=0,500 s/mm2), lipid concentrations within three voxels (~1cm³) positioned over muscle fibers predominantly oriented in the foot-head (tibialis anterior/TA), right-left (Soleus-Solred), and anterior-posterior (Soleus-Solgrn) were ascertained using MRS(T₂ corrected PRESS). Further, orthogonal thin slice maximum intensity projection of Dixon derived fat images were used to avoid voxel positioning over interstitial or fascial fat. Data Analysis: Pennation angle of muscle fibers with B₀ was calculated from the eigenvector (ε�) of DTI fiber orientation (Φ=acos(ε�.[0,0,1]). The chemical shift (δ) of EMCL with respect to IMCL, and lipid concentrations were quantified using JMRUI-v5.
Results: Compared to TA muscle group,Φ was substantially larger and more variable for voxels placed over soleus muscle. Normalized lipid concentrations of IMCL/water and EMCL/water were: 0.23±0.13%,1.05±0.56%,0.91±0.29% and 0.83±0.6%,1.69±0.91%,2.79±1.86% for TA, Solred and Solgrn respectively. In contrast to IMCL, Bland-Altman analysis revealed significant variation in EMCL/water (bias: 0.53%, SD: 1.72%) between voxels positioned over Solred and Solgrn. The δ of EMCL was 0.23±0.02ppm,0.20±0.01ppm, and 0.18±0.03ppm within TA, Solred, and Solgrn respectively and corresponded to theoretical predictions.
Conclusion: In soleus muscle group, EMCL concentrations determined by MRS are significantly influenced by muscle fiber orientation. In longitudinal studies, DTI derived fiber orientation, and Dixon derived fat maps, can help position MRS voxels consistently between imaging sessions.
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