Assessment of lipids in skeletal muscle by LCModel and AMARES

Abstract

Purpose: To process single voxel spectra of the human skeletal muscle by using an advanced method for accurate, robust, and efficient spectral fitting (AMARES) and by linear combination of model spectra (LCModel). To determine absolute concentrations of extra-(EMCL) and intramyocellular lipids (IMCL). Materials and Methods:Single-voxel proton magnetic resonance spectroscopy (PRESS) was used to obtain the spectra of the calf muscles. Unsuppressed water line was used as a concentration reference. A new prior knowledge for AMARES was proposed to estimate the concentrations of EMCL and IMCL. The prior knowledge was derived from the spectrum of vegetable oil. The results were compared with the values estimated by LCModel. Absolute concentrations of total lipid content in millimoles per kilogram wet weight were used for the comparisons.Results: Absolute concentrations of total lipid content in skeletal muscle were estimated by AMARES and LCModel. Very good correlation of the total fat (EMCL ϩ IMCL) and IMCL concentrations was achieved between both data processing approaches. The absolute quantification of EMCL and IMCL based on MRS data is not a trivial problem. It depends on the ability to distinguish the methylene spectral line of IMCL (IMCL CH2 ) from the methylene line of EMCL (EMCL CH2 ), on relaxations corrections and on the accuracy of the biochemical constants that allow the conversion of the EMCL CH2 and IMCL CH2 spectral intensities to the absolute concentration expressed as millimoles per kilogram wet weight (mmol/kg ww). Because of these difficulties, relatively few research groups reported lipid concentrations in the absolute units (6,(11)(12)(13)(14). The majority of these studies avoided absolute quantification and used the relative measures fat-to-total creatine (creatine and phosphocreatine at ϳ3 ppm) ratio (15-18) or fat-to-unsuppressed water ratio (2,3,10,19,20), where the word fat represents EM-CL CH2 and IMCL CH2 spectral intensities. The applicability of these measures is based on the assumption that water in the muscles or total creatine concentration remains constant during the whole study. These not always valid assumptions and the problem of relaxation corrections can be avoided by using fat as the internal or external concentration reference. Yellow bone marrow (5,8,14,21) and vegetable oil (14) were used for this purpose. An advantage of fat reference is that it does not vary and that the conversion to absolute concentration is straightforward if the same lipid composition is assumed (22). ConclusionSpectral overlap with EMCL is probably the most important factor that limits accuracy of IMCL estimation. It has been shown that sophisticated fitting methods are necessary to differentiate IMCL besides overlap-

In vivo high-resolution magic angle spinning magnetic resonance spectroscopy of Drosophila melanogaster at 14.1 T shows trauma in aging and in innate immune-deficiency is linked to reduced insulin signaling

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Chronic Reduction of Plasma Free Fatty Acid Improves Mitochondrial Function and Whole-Body Insulin Sensitivity in Obese and Type 2 Diabetic Individuals

“…Spectral data were analyzed using the jMRUI software (17) using prior knowledge values for the AMARES quantitation package (18) as developed by Weis et al (19) to distinguish the extramyocellular (EMCL) - CH 2 spectral line from the IMCL-CH 2 line. Baseline correction was performed by truncating the first two points of the free induction decay and by applying the time domain Hankel-Lanczos Singular Value Decomposition filter to remove the underlying tails from the residual water resonance.…”

“…The equation of Szczepaniak et al (21) was used to calculate the absolute concentrations expressed as millimoles per kilogram of wet weight from the methylene-to-water spectral intensity ratio (Z). If LC is the lipid content in millimoles per kilogram wet weight, then LC = (ZW × 10 6 ) ÷ [885.4 D (ZW + P)], where W = 0.76 represents the tissue water content relative to total weight (kg/kg) of the normal muscle tissue, T = 1.024 is the weighted density of the fat (triglyceride fatty acids) relative to the triolein standard (molecular weight 885.4) (21), D = 1.05 kg/L is the density of lean muscle tissue, and P = 0.61 is the relative methylene proton density (mol/mol) of tissue fat versus water (19). …”

Different quantification algorithms may lead to different results: a comparison using proton MRS lipid signals

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