Investigation of adipose tissues in Zucker rats using in vivo and ex vivo magnetic resonance spectroscopy

Abstract

This article is available online at http://www.jlr.org ties ( 1-7 ), highlighting the need for effi cient means of characterizing lipid composition in vivo ( 1-7 ). Parameters such as mean unsaturation, mean polyunsaturation, and mean chain length can provide useful information about diet, fat distribution, and metabolism ( 9-14 ). The abovementioned parameters of lipid deposits can be obtained in vitro using high resolution proton magnetic resonance spectroscopy (HR-NMR) (15)(16)(17)(18)(19)(20). The disadvantages associated with in vitro techniques are well known: invasiveness and time-consuming extraction protocols ( 21 ). The main advantage is high resolution, which allows discrimination between olefi nic proton peaks and glycerol methilenic peaks.To the best of our knowledge, ours was one of the fi rst groups to suggest that in vivo magnetic resonance spectroscopy (MRS) can provide information about fat composition in living animals ( 22 ). In that paper, chemical shift imaging was used to obtain parametric maps of PUFA distribution in adipose tissue of living rats. Recently, other groups have proposed in vivo single-voxel localized spectroscopy for the characterization of lipid tissue in animals ( 23 ) and humans ( 24 ) at 7 T. Single-voxel techniques take advantage of the possibility to select a relatively small volume of interest (of the order of 1-2 mm 3 in animals) that allows to obtain good voxel-based shimming and consequently, well-resolved resonances ( 23 ).However, with optimized single-voxel techniques in vivo spectra are also affected by relatively high line-width with partially overlapping peaks. Reliable extraction of lipid parameters from in vivo spectra requires suitable methods for spectral analysis and processing ( 23, 24 ). Lipid parameters are calculated from the relationships between the Abstract In vivo single-voxel magnetic resonance spectroscopy (MRS) at 4.7T and ex vivo high-resolution proton magnetic resonance spectroscopy (HR-NMR) at 500 MHz were used to study the composition of adipose tissues in Zucker obese and Zucker lean rats. Lipid composition was characterized by unsaturation and polyunsaturation indexes and mean chain lengths. In vitro experiments were conducted in known mixtures of triglycerides and oils in order to validate the method. To avoid inaccuracies due to partial peak overlapping in MRS, peak quantifi cation was performed after fi tting of spectral peaks by using the QUEST algorithm. The intensity of different spectral lines was also corrected for T2 relaxation. Albeit with different sensitivity and accuracy, both techniques revealed that white adipose tissue is characterized by lower unsaturation and polyunsaturation indexes in obese rats compared with controls. HR-NMR revealed similar differences in brown adipose tissue. The present fi ndings confi rm the hypothesis that obese and lean Zucker rats have different adipose tissue composition.

Effect of J‐coupling on lipid composition determination with localized proton magnetic resonance spectroscopy at 9.4 T

“…For example, in the case of corn oil the calculated amount of linoleic acid is about 17.5% less than the lowest expected value of 50.4% and the estimated amount of oleic acid is %45.7% higher than the highest expected value of 34.6%. Mosconi et al (12) determined oil and rat adipose tissue compositions using high-resolution NMR and using PRESS spectra corrected for an apparent T 2 . It was observed from their results that the obtained polyunsaturation indices from the PRESS spectra were also underestimated in both oils and rats compared with those calculated from the high-resolution spectra.…”

“…It has been suggested that there may be a link between the composition of fatty tissue and the risk of suffering from such diseases (11,12). The amounts of saturated, monounsaturated, and polyunsaturated fatty acids composing a lipid can be determined from a high-resolution NMR (nuclear magnetic resonance) spectrum of the sample.…”

“…Furthermore, all lipid protons are involved in scalar coupling (J-coupling) interactions (21)(22)(23), which cause their response to PRESS to be modulated as a function of TE. Efforts have been made to correct for T 2 losses (11,12,18); however, to our knowledge signal correction for J-coupling evolution has not been performed when establishing lipid compositions using in vivo sequences. To demonstrate the influence of scalar coupling effects, we compare lipid compositions obtained from short-TE PRESS spectra, from short-TE PRESS spectra corrected for T 2 relaxation (18), and from spectra acquired with a PRESS sequence designed to minimize the effects of J-coupling (24,25).…”

“…High-resolution NMR has been employed to identify the constituents of oils, animal fats, and extracts of rat adipose tissue (13)(14)(15)(16). Recently, in vivo MRS measurements of rodents (12,17,18) and humans (11) have been performed at high field strengths to calculate adipose tissue composition from the areas of the 2.1 ppm, 2.3 ppm, and 2.8 ppm (diallylic protons) proton lipid peaks. Previous studies (11,12,18) utilized the popular PRESS (point resolved spectroscopy) (19,20) pulse sequence for spatial localization.…”

“…Recently, in vivo MRS measurements of rodents (12,17,18) and humans (11) have been performed at high field strengths to calculate adipose tissue composition from the areas of the 2.1 ppm, 2.3 ppm, and 2.8 ppm (diallylic protons) proton lipid peaks. Previous studies (11,12,18) utilized the popular PRESS (point resolved spectroscopy) (19,20) pulse sequence for spatial localization. During the PRESS echo time, TE, the signal of each lipid resonance decays due to transverse relaxation according to a T 2 constant characteristic of the protons contributing to that peak.…”