Intramyocellular lipid dependence on skeletal muscle fiber type and orientation characterized by diffusion tensor imaging and ¹H-MRS

When muscle fibers are aligned with the B₀ field, intramyocellular lipids (IMCL), important for providing energy during physical activity, can be resolved in proton magnetic resonance spectra ( ¹H-MRS). Various muscles of the leg differ significantly in their proportion of fibers and angular distribution. This study determined the influence of muscle fiber type and orientation on IMCL using ¹H-MRS and diffusion tensor imaging (DTI). Muscle fiber orientation relative to B ₀ was estimated by pennation angle (PA) measurements from DTI, providing orientation-specific extramyocellular lipid (EMCL) chemical shift data that were used for subject-specific IMCL quantification. Vastus lateralis (VL), tibialis anterior (TA) and soleus (SO) muscles of 6 healthy subjects (21-40 yrs) were studied on a Siemens 3T MRI system with a flex 4-channel coil. ¹H-MRS were acquired using stimulated echo acquisition mode (STEAM, TR=3s, TE=270ms). DTI was performed using single shot EPI (b=600s/mm ², 30 directions, TR=4.5s, TE=82ms, and ten×5mm slices) with center slice indexed to the MRS voxel. The average PA's measured from ROI analysis of primary eigenvectors were PA=19.46±5.43 for unipennate VL, 15.65±3.73 for multipennate SO, and 7.04±3.34 for bipennate TA. Chemical shift (CS) was calculated using [3cos²θ-1] dependence: 0.17±0.02 for VL, 0.18±0.01 for SO and 0.19±0.004 ppm for TA. IMCL-CH₂ concentrations from spectral analysis were 12.77±6.3 for VL, 3.07±1.63 for SO and 0.27±0.08 mmol/kg ww for TA. Small PA's were measured in TA and large CS with clear separation between EMCL and IMCL peaks were observed. Larger variations in PA were measured VL and SO resulting in an increased overlap of the EMCL on IMCL peaks.