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.