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

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. A 1H magnetic resonance spectroscopy PRESS sequence is optimized for resolving the ω‐3 fatty acid methyl resonance from that of non‐ω‐3 methyl protons at 9.4 T by using a TE of 109 ms (TE1 = 15 ms). The efficacy of the sequence was verified on visceral adipose tissue of three groups of mice, each fed a diet with different ω‐3 content.