Diffusion-weighted magnetic resonance spectroscopy: a novel approach to investigate intramyocellular lipids

Motivated by the potential difference between intramyocellular lipid (IMCL) and extramyocellular lipid (EMCL) in their microscopically restricted diffusion environments, proton diffusion properties in IMCL and EMCL were characterized by diffusion-weighted magnetic resonance spectroscopy. Ex vivo experiments were conducted on fresh pig lower hindlimb muscle samples. In vivo experiments were performed on the lower hindlimbs of normal adult Sprague-Dawley rats. Ex vivo apparent diffusion coefficients at 20°C were found to be in the ranges of (10.8 ± 0.5) × 10(-6) to (15.8 ± 0.8) × 10(-6) mm(2)/s for EMCL and (1.6 ± 0.6) × 10(-6) to (2.4 ± 0.7) × 10(-6) mm(2)/s for IMCL depending on the diffusion weighting direction relative to muscle fiber orientation. In vivo apparent diffusion coefficients of EMCL and IMCL were measured as (13.8 ± 0.9) × 10(-6) and (4.6 ± 0.7) × 10(-6) mm(2)/s, respectively, along the direction perpendicular to muscle fiber orientation. Our results demonstrated that the apparent diffusion coefficient of EMCL was substantially higher than that of IMCL. Therefore, EMCL could be effectively suppressed by proper diffusion weighting to achieve reliable detection of IMCL despite unknown or/and multiple muscle fiber orientations. Such lipid proton diffusion approach can be used for robust IMCL separation and identification. Furthermore, it may provide new biophysical insights in the investigation of lipid metabolism in obesity and diabetes.