Orientation of lipid strands in the extracellular compartment of muscle: Effect on quantitation of intramyocellular lipids

Single‐voxel 1 H NMR spectra from gastrocnemius and soleus muscle were acquired in healthy volunteers at 7T with the objective of measuring the concentration of intramyocellular lipid [IMCL] (note: throughout this article, square brackets indicate concentration). However, significant asymmetry in the resonance assigned to the methylene protons (‐CH 2 ‐) n in extramyocellular lipids (EMCL) interfered with fitting the spectra. Since muscle fibers in these tissues are generally not parallel to B 0 , the influence of variable orientation in strands of extracellular fat was examined using a mathematical model. Modest variation in orientation produced asymmetric lineshapes that were qualitatively similar to typical observations at 7T. Analysis of simulated spectra by fitting with a Voigt function overestimated [IMCL]/[EMCL] except when EMCL fibers were nearly parallel to B 0 . Estimates of [IMCL]/[EMCL] were improved by including variations in fiber orientation in the lineshape analysis (fiber orientation modeling, or FOM). Calculated [IMCL] using FOM, 4.8 ± 2.2 mmol/kg wet weight, was lower compared to most previous reports in soleus. Magn Reson Med 61:16–21, 2009. © 2008 Wiley‐Liss, Inc.