Fiber orientation measurements by diffusion tensor imaging improve hydrogen-1 magnetic resonance spectroscopy of intramyocellular lipids in human leg muscles

Twelve healthy subjects underwent hydrogen-1 magnetic resonance spectroscopy (1H-MRS) acquisition (15 × 15 × 15 mm3), diffusion tensor imaging (DTI) with a b-value of 600 smm^-2, and fat-water magnetic resonance imaging (MRI) using the Dixon method. Subject-specific muscle fiber rientation, derived from DTI, was used to estimate the lipid proton spectral chemical shift. Pennation angles were measured as 23.78 deg in vastus lateralis (VL), 17.06 deg in soleus (SO), and 8.49 deg n tibialis anterior (TA) resulting in a chemical shift between extramyocellular lipids (EMCL) and ntramyocellular lipids (IMCL) of 0.15, 0.17, and 0.19 ppm, respectively. IMCL concentrations were .66 ± 1.24 mmol kg^-1, 6.12 ± 0.77 mmol kg^-1, and 2.33 ± 0.19 mmol kg^-1 in SO, VL, and TA, espectively. Significant differences were observed in IMCL and EMCL pairwise comparisons in SO, L, and TA (p ≤ 0.05). Strong correlations were observed between total fat fractions from 1H-MRS and Dixon MRI for VL (r = 0.794), SO (r = 0.655), and TA (r = 0.897). Bland-Altman analysis between at fractions (FFMRS and FFMRI) showed good agreement with small limits of agreement (LoA): bias = - .21% (LoA: -1.12% to 0.69%) in VL, bias = 0.025% (LoA: -1.28% to 1.33%) in SO, and bias = - .13% (LoA: -0.74% to 0.47%) in TA. The results of this study demonstrate the variation in muscle fiber orientation and lipid concentrations in these three skeletal muscle types.