Regional differences in intramyocellular lipids in humans observed by in vivo ¹H-MR spectroscopic imaging

Abstract

Regional differences in the content of intramyocellular lipids (IMCL), extramyocellular lipids, and total creatine (TCr) were quantified in soleus (S), tibialis posterior (TP), and tibialis anterior (TA) muscles in humans using in vivo 1H proton spectroscopic imaging at 4 T. Improved spatial resolution (0.25-ml nominal voxel resolution) made it feasible to measure IMCL in S, TP, and TA simultaneously in vivo. The most significant regional difference was found in the content of IMCL compared with extramyocellular lipids or TCr. The concentrations of TCr were found to be 29-32 mmol/kg, with little regional variation. IMCL content was measured to be 4.8 +/- 1.6 mmol/kg tissue wt in S, 2.8 +/- 1.3 mmol/kg tissue wt in TP, and 1.6 +/- 0.9 mmol/kg tissue wt in TA in the order of S > TP > TA (P < 0.05). It is likely that these IMCL values are consistent with the known fiber types of these muscles, with S having the greatest fraction of type I (slow-twitch, oxidative) fibers and TA having a large fraction of type IIb (fast-twitch, glycolytic) fibers.

Role of proton MR for the study of muscle lipid metabolism

“…The concentration of creatine is rather stable in muscular tissue (approximately 30 mmol/kg wet weight) and shows little inter-individual and regional variations (97)(98)(99), despite the fact that creatine concentration changes slightly with nutrition (100,101). In addition, creatine signals did not vary before and for sufficiently long period after exercise in an MR study (65).…”

“…In recent years, 1 H-MR spectra of IMCL have been acquired at field strengths above 3 T, either in humans (52,98,115) or particularly in small animals (23,25,27,28). These studies show the potential of higher field strengths for a separation of IMCL and EMCL, and particularly the reduction of contamination by EMCL that can be achieved by a smaller voxel size.…”

“…Such a selection of voxels during postprocessing is obviously impossible in single voxel techniques. MRSI of human skeletal muscle has been used successfully by several groups, either as full images (69,115,118,119) or as selected voxels from an MRSI data set (52,98).…”

Inductively coupled helmholtz coil on a dedicated imaging platform for the in vivo1H‐MRS measurement of intramyocellular lipids in the hind leg of rats

“…Compared to a standard surface coil, the Helmholtz coil platform provides a homogenous radiofrequency field profile and significantly increased imaging SNR, translating directly into an improved SNR in single voxel MR spectroscopy (ϩ80%) and a decreased experimental time (-43%). In case a decreased measurement time is not needed, an improved SNR will allow the user to achieve a more precise, quantitative analysis of the data (reduced noise), to reduce the voxel's volume in order to assess even smaller regional variations, or to perform chemical shift imaging in the muscle (19,20). In addition to its advantageous imaging and spectroscopy properties, handling of the animal and fixation of the leg are markedly improved using the Helmholtz coil platform.…”

“…By design, surface coils exhibit a hemispheric radiofrequency field profile, which directly translates into a distance-dependent reception profile. However, especially for the measurement of regional differences (12) or spectroscopic imaging (13) in rat muscles, a homogeneous reception profile is highly desirable. In addition, an increase in sensitivity of the receiving coil would allow the user to either shorten the measuring time for higher throughput or reduce the size of the voxel volume for higher spatial specificity within the muscle.…”