Figure 1
1H-MRS intramyocellular lipid (IMCL): the CH3:CH2 ratio as a compositional marker in theory, in phantoms, and in vivo. (A) Theory. The figure shows the principle of the CH3:CH2 ratio as a compositional marker in IMCL (which detected by 1H MRS is overwhelmingly triglyceride (TG)34), influenced primarily by the degree of fatty acid (FA) unsaturation, and secondarily by FA chain length. The upper panel shows the palmitic acid component of a TG molecule with the theoretical ratio of 3/24 = 0.125 of CH3 (at 0.9 ppm) to CH2 (at 1.3 ppm). The middle panel shows how introduction of a single double bond increases this ratio to 3/16 = 0.188, due not only to the desaturation of two CH2, but also to the alteration in the chemical environment of the neighbouring CH2. For FA chains of equal number of double bonds, the CH3:CH2 ratio will also be scaled by chain length, although this will have a proportionally smaller effect (lower panel). SFA, saturated FA; MUFA, monounsaturated FA; PUFA, polyunsaturated FA. (B) Results in phantoms. The graph on the right of the panel shows data from agar phantoms simulating IMCL (oil droplets) and extramyocellular lipid (EMCL; oil-soaked tissue roll) made using olive, rapeseed, sunflower and cod liver oils. Three 1H spectra were acquired from each phantom and the IMCL CH3:CH2 ratio correctly distinguished the order of four oil phantoms. On the left of the panel is a representative 1H spectrum (solid black) acquired from a voxel containing both simulated IMCL and EMCL compartments, also showing the overall fit (dashed grey) and individual fit components (solid grey). (C) Results in vivo. The figure shows a representative 1H spectrum from soleus in vivo, showing the fit to EMCL and IMCL CH2 and CH3 resonances.
