Resolving the omega‐3 methyl resonance with long echo time magnetic resonance spectroscopy in mouse adipose tissue at 9.4 T

Abstract

Tissue omega‐3 (ω‐3) content is biologically important to disease; however, its quantification with magnetic resonance spectroscopy in vivo is challenging due to its low concentration. In addition, the ω‐3 methyl resonance (≈ 0.98 ppm) overlaps that of the non‐ω‐3 (≈ 0.90 ppm), even at 9.4 T. We demonstrate that a Point‐RESolved Spectroscopy (PRESS) sequence with an echo time (TE) of 109 ms resolves the ω‐3 and non‐ω‐3 methyl peaks at 9.4 T. Sequence efficacy was verified on five oils with differing ω‐3 fat content; the ω‐3 content obtained correlated with that measured using 16.5 T NMR (R2 = 0.97). The PRESS sequence was also applied to measure ω‐3 content in visceral adipose tissue of three different groups (all n = 3) of mice, each of which were fed a different 20% w/w fat diet. The fat portion of the diet consisted of low (1.4%), medium (9.0%) or high (16.4%) ω‐3 fat. The sequence was also applied to a control mouse fed a standard chow diet (5.6% w/w fat, which was 5.9% ω‐3). Gas chromatography (GC) analysis of excised tissue was performed for each mouse. The ω‐3 fat content obtained with the PRESS sequence correlated with the GC measures (R2 = 0.96). Apparent T2 times of methyl protons were assessed by obtaining spectra from the oils and another group of four mice (fed the high ω‐3 diet) with TE values of 109 and 399 ms. Peak areas were fit to a mono‐exponentially decaying function and the apparent T2 values of the ω‐3 and non‐ω‐3 methyl protons were 906 ± 148 and 398 ± 78 ms, respectively, in the oils. In mice, the values were 410 ± 68 and 283 ± 57 ms for ω‐3 and non‐ω‐3 fats, respectively.

Analytical methods used for the characterisation of specific features of biological tissues related with obesity: A review

“…The body composition was determined by NMR spectroscopy. 1 H, 13 C and 31 P NMR spec-troscopies were used to investigate different health problems related with obesity performing animal experiments with male rats [115] and adipose tissue of mouse [116,117]. High resolution magic angle spinning NMR spectroscopy was successfully used for the qualitative study of the highly specialised lipid tissues of cetaceans [118].…”

“…α-methylene to double bond and terminal methyl function [115,116,117] Long-finned pilot whale inner blubber…”

Echo time optimization for in‐vivo measurement of unsaturated lipid resonances using J‐difference‐edited MRS

“…Conventional 1 H MRS can assess lipid signals noninvasively, but its limited accuracy for measuring unsaturated lipid functional groups prohibits the probing of lipid compositions in vivo. 13,14 There are multiple ways to evaluate the lipid composition by conventional MRS. Lundbom et al defined the unsaturation index (UI) as S vinyl /(S vinyl + S methylene ) to evaluate lipid unsaturation of human white adipose tissue (WAT) and liver fat, where S x denotes the signal intensity of the x group in spectra (Figure 1). 15 UI could also be calculated in a simplified version as S vinyl /S methyl , while the polyunsaturation index (PUI) was defined as S diallylic /S methyl .…”