Measurement of intracellular triglyceride stores by H spectroscopy: validation in vivo

TLDR

The study aims to validate 1H MRS as a noninvasive method to quantitatively differentiate adipocyte from intracellular triglyceride stores by monitoring methylene proton signals at 1.6 and 1.4 ppm. The authors applied 1H MRS to measure methylene signals at 1.6 and 1.4 ppm in two animal models of intracellular TG accumulation, confirming histological evidence of intrahepatic and intramyocellular TG deposition. Findings show that 1H MRS reliably detects intracellular TG via the 1.4‑ppm methylene signal, with signal intensity correlating strongly with histology and biochemical TG levels (R = 0.934), and that the technique distinguishes intramyocellular from extramyocellular fat with acceptable precision (CV ≈ 8–12 % for intramyocellular and 23–53 % for extramyocellular), confirming its feasibility and accuracy at 1.5‑T field strengths.

Abstract

We validate the use of 1 H magnetic resonance spectroscopy (MRS) to quantitatively differentiate between adipocyte and intracellular triglyceride (TG) stores by monitoring the TG methylene proton signals at 1.6 and 1.4 ppm, respectively. In two animal models of intracellular TG accumulation, intrahepatic and intramyocellular TG accumulation was confirmed histologically. Consistent with the histological changes, the methylene signal intensity at 1.4 ppm increased in both liver and muscle, whereas the signal at 1.6 ppm was unchanged. In response to induced fat accumulation, the TG concentration in liver derived from 1 H MRS increased from 0 to 44.9 ± 13.2 μmol/g, and this was matched by increases measured biochemically (2.1 ± 1.1 to 46.1 ± 10.9 μmol/g). Supportive evidence that the methylene signal at 1.6 ppm in muscle is derived from investing interfascial adipose tissue was the finding that, in four subjects with generalized lipodystrophy, a disease characterized by absence of interfacial fat, no signal was detected at 1.6 ppm; however, a strong signal was seen at 1.4 ppm. An identical methylene chemical shift at 1.4 ppm was obtained in human subjects with fatty liver where the fat is located exclusively within hepatocytes. In experimental animals, there was a close correlation between hepatic TG content measured in vivo by 1 H MRS and chemically by liver biopsy [ R = 0.934; P < .0001; slope 0.98, confidence interval (CI) 0.70–1.17; y-intercept 0.26, CI −0.28 to 0.70]. When applied to human calf muscle, the coefficient of variation of the technique in measuring intramyocellular TG content was 11.8% in nonobese subjects and 7.9% in obese subjects and of extramyocellular (adipocyte) fat was 22.6 and 52.5%, respectively. This study demonstrates for the first time that noninvasive in vivo 1 H MRS measurement of intracellular TG, including that within myocytes, is feasible at 1.5-T field strengths and is comparable in accuracy to biochemical measurement. In addition, in mixed tissue such as muscle, the method is clearly advantageous in differentiating between TG from contaminating adipose tissue compared with intramyocellular lipids.