Lipid content in the musculature of the lower leg: Evaluation with high‐resolution spectroscopic imaging

Abstract

A novel spectroscopic imaging method with high spectral and spatial resolution was developed for the specific goal of assessing muscle fat. Sensitivity to the methylene and methyl protons of fatty acids was improved by the use of a binomial 1 1 excitation pulse instead of the standard radiofrequency (RF) pulse. Acceptable measurement time is achieved by using a narrow spectral bandwidth (6 ppm). The spectral resolution is sufficient to resolve extramyocellular (EMCL) and intramyocellular ( Key words: spectroscopic imaging; narrow bandwidth; muscular lipids; fat; intramyocellular triglyceridesThe first assessments of muscle fat were performed in biopsy specimens. However, because of the invasive nature of this technique, only a few data points can be obtained. Computed tomography (CT) has been used both to measure the total body fat content and to study different fat compartments, and provides a relatively accurate measurement of internal and subcutaneous fat (1,2). However, the low lipid concentrations in muscle cannot be quantified accurately from biopsy specimens or by CT because neither of these techniques is sufficiently sensitive to distinguish between extramyocellular (EMCL) and intramyocellular (IMCL) lipids. These problems can be overcome with the use of 1 H magnetic resonance spectroscopy (MRS) (3), which is a unique noninvasive method with high sensitivity. The main metabolites that are detectable in proton spectra of muscles include EMCL, IMCL, trimethylammonium, creatine/phosphocreatine, carnosine, and taurine (4).It is important to assess EMCL and IMCL stores in skeletal musculature in studies of the physiological and pathological aspects of lipid metabolism (5-7). Bulk fat (EMCL) is stored subcutaneously and in interstitial (adipose) tissue. Along the fasciae, muscle boundaries and fibers form macroscopic plate structures that run almost parallel with the long axes of the extremities. Intramyocellular lipids are stored in the form of liquid droplets in the cytoplasm of muscle cells (8). Different muscle groups contain different amounts of EMCLs and IMCLs (7,9), and these amounts are considerably greater in obese than in lean individuals (6,7). EMCLs serve as a long-term fat depot, whereas IMCLs are involved in lipid metabolism with a turnover of some hours. IMCLs vary in quantity during physical exercise (8) and also depend on food intake (10,11). The role of IMCL in diabetes is not fully understood; however, an inverse correlation between IMCL concentration and insulin sensitivity has been confirmed by several research groups (6,7,12,13).The spectral line positions of EMCL and IMCL depend on both the chemical shifts and the local bulk magnetic susceptibility effects. Since the IMCL droplets have a spherical shape and are stored in a susceptibility homogeneous (ϳwater) environment of muscle cells, IMCL signals are independent of the muscle orientation relative to the magnetic field, B 0 . IMCL always resonates at the same frequency: 1.3 ppm. For macroscopic EMCL structures distributed along muscle...

Technical evaluation of in vivo abdominal fat and IMCL quantification using MRI and MRSI at 3 T

“…Reproducibility of MRS in TA is slightly better than that in SO (average CV of 11.9% in TA and 13.7% in SO). This can probably be explained by two facts: 1) the muscle fibers in TA are oriented approximately parallel to B 0 (approximately 9°) while those in SO are approximately 45° relative to B 0 (16), and therefore the separation between EMCL and IMCL is larger in TA; 2) the total fat content, especially in the EMCL compartment, is greater in SO than that in TA (17). Thus the spectral signal is more dominated by EMCL in SO, potentially affecting the accuracy of IMCL estimation.…”

“…However, it is not entirely clear whether IMCL measurement is more reliable in the primarily slow-twitch soleus muscle (SO) or the relatively fast-twitch tibialis anterior (TA). Moreover, reports of 2D or 3D MR spectroscopic imaging (MRSI) techniques to evaluate muscle lipids and their distribution in different muscle groups have generally come from studies employing 1.5 Tesla (T) (16,17). With recent developments in high field MR, such as the availability of clinical systems with a field strength of 3T, it is interesting to investigate the feasibility of using MRS or MRSI to evaluate IMCL at 3T.…”

Role of proton MR for the study of muscle lipid metabolism

“…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).…”

Stability and sensitivity of waterT2obtained with IDEAL‐CPMG in healthy and fat‐infiltrated skeletal muscle

“…Rather, a pragmatic approach was used to estimate the proton‐density at t  = 0 by fitting a mono‐exponential function to the fat signal decay. The spectrum of fat has multiple components,35 each with a different characteristic decay time 4. The dominance of a long T 2 ‐decay component, which is often observed in the fat signal decay, is most likely ascribable to the large constant offset c f .…”