Human Muscle Studies by Magnetic Resonance Spectroscopy

Magnetic resonance spectroscopy (MRS) of human skeletal muscle has contributed significantly to our knowledge of muscle physiology but its application in daily clinical routine is not as widespread as previously expected. One of the reasons is the lack of broadband capabilities of whole-body MR systems, which are necessary for the study of 13C- or 31P, the two nuclei that play an important role in MRS of skeletal muscle. While 13C-MRS allows studies of muscle glycogen and 13C-labeled substances, 31P-MRS shows signals from high-energy phosphates and metabolites from membrane lipids. 1H-MRS came later into play as it was previously assumed that the large lipid signals would hide low-concentration metabolites. However, the effect of spatial ordering in skeletal muscle gives 1H-MRS unique advantages, and, in particular, the separation of intramyocellular (IMCL) from extramyocellular lipids (EMCL) based on susceptibility effects has made 1H-MRS an important tool in studies on insulin resistance and the metabolic syndrome. The future of MRS of human skeletal muscle will include an increasing number of combined studies of 1H-, 13C-, 31P-MRS, and MRI, because they allow for a comprehensive and noninvasive observation of muscle metabolism. However, this will strongly depend on the future availability of multinuclear whole-body MR systems.