We present a method for detecting omega-3 fatty acids (FA) at 7T by ¹H-MR spectroscopy (MRS) using a MEGA-sLASER editing sequence[1][2]. The current consent is that omega-3 FA play an important role in metabolism since they cannot be synthesized by the human body. As previous studies showed, supplementation of omega-3 FA may decreases the risk of major depression[3] and reduces the risk of cardiovascular disease [4]. Currently only two publications[5,6] about in vivo detection of omega-3 FA using ¹H-MRS are known to us, one 1.5T and one 3T study.

The CH₂ (methylene) groups of omega-3 FA and non-omega-3 FA resonate at 2.1ppm and 1.3ppm, respectively, because of the existence of a double-bond between the third and fourth carbon atom in the FA chain. The CH₃ (methyl) signal at 0.9ppm remains the same in both cases. According to Skoch et al.[5] the the omega-3 FA signal can be described by a CH₂-CH₃ system. Because of magnetically equivalent CH₃ protons the spin system is of the form A₂X where X and its corresponding spin operator represent the signal of interest.

The J-modulation according to[5,6] has been assumed for the MEGA-sLASER single-voxel sequence and was validated in phantom (Menhaden fish oil) measurements for echo times up to 1500ms. The formula for a pure antiphase state[5] reads TE=(1/2+k)/J for k=0,1,2,... with J=7.52Hz. These antiphase modulations are obtained by setting the MEGA suppression pulses to an off resonance frequency of 0.14ppm. For selective refocusing the on resonance pulses radiated the CH₂ signal at 2.1ppm. Concerning the in vivo measurements the values TE=332ms and 465.4ms were verified as most optimal.

All experiments were carried out on a 7 T human MR Scanner (Siemens Healthcare) with five healthy volunteers (1 female), using a ¹H-28-channel knee coil. 2×2×2cm³ voxels were placed in the adipose tissue of the thigh (Fig. 1), readout BW 3000Hz, shimming performed on the CH₂ resonance at 0.9ppm with FWHM≈40Hz for in vivo measurements. The MEGA-sLASER sequence consist of a 2.6ms 90^° symmetrical SLR excitation pulse and four 5ms 180^° AFP GOIA-WURST(16,4) pulses, BW 12kHz, for localization and two 80ms symmetrical SLR pulses, BW 100Hz, for MEGA editing/selective refocusing. Subtractions of on and off resonances after phase and frequency correction were done with the jMRUI software package[7].

On Resonance: The omega-3 FA methyl signal appeared in oil as a triplet with intensity ratios of 1:2:1 at the positions 1ppm:0.975ppm:0.951ppm. Off resonance: The outer peaks of the triplet point down forming a -1:2:-1 pattern. The subtraction of both results in a 2:0:2 pseudo-doublet as seen according to the phantom measurement in Fig.2 (1 average). Comparison of in vivo on- and off-resonance from volunteer 1 are shown in Fig.3. for echo time TE=365.4ms. The difference spectrum is illustrated in Fig.4 (magenta), together with the difference spectra for echo time TE=332ms and of a second volunteer for comparison (16 averages). The left peak of the omega-3 FA triplet is marked by an arrow at approximately 0.995ppm. The imperfect subtraction of CH₃ and omega-3 CH₃ is visible but both signals can be separated using adequate prior knowledge during spectral fitting. The results of in vivo measurements with an ultra long echo time of TE=1130ms (32 averages) are visible in Fig.5 and show that there is still omega-3 CH₃ signal left, while the CH₂ resonance at 1.3ppm is completely suppressed. Due to the very high pulse bandwidth of the localization pulses the 4-compartment effect was greatly reduced compared to PRESS and STEAM. Considering the coupling partners to be at 2.1ppm and 1ppm, the following portion of the VOI was correctly edited: sLASER-97.3%, PRESS-72.8% and STEAM-79.6% (for one dimension).

MEGA editing is a powerful tool for quantification of metabolites in overlapping regions. More over, the effects of chemical shift evolutions are refocused during the localization due to the symmetrical implementation of the frequency selective MEGA pulses around the localization pulses. The combination of MEGA editing the sLASER localization offers reduced sensitivity to B₁-inhomogeneities and lower pulse power requirements compared to PRESS or STEAM. Also sLASER substantially reduces the 4-compartment effect, which reduces editing efficiency, and which is caused by the large chemical shift displacement errors at 7T. Imperfect subtraction (as seen in Fig.4) of spectra may arise from small frequency and phase changes due to subject motion or scanner instabilities and have to be corrected during data processing. We showed that the in vivo detection of omega-3 at 7T provides improved results because of better spectral resolution. This makes MEGA-sLASER a powerful non-invasive tool for rapid and repeated use in longitudinal studies targeting changes in tissue omega-3 FA levels.