Proton MRS of changes of lipid unsaturation during liver regeneration

1 H MRS experiments were performed on a 7T Bruker MRI scanner with a 60 mm RF Tx/Rx quadrature resonator. Animals were anesthetized with ~1.5 % isoflurane, and body temperature was maintained at about 36.5°C with respiratory monitoring. Scout images were first acquired in three orthogonal planes with a FLASH sequence. A 5×5×5 mm 3 voxel was placed on a homogeneous liver parenchyma, without involving large blood vessels. MAPSHIM protocol was applied for shimming, and each voxel was shimmed by the first- and second-order localized voxel shimming based on field map technique. A FWHM linewidth of water signal of <30 Hz was achieved. The water signal was suppressed by variable power RF pulses with optimized relaxation delays (VAPOUR). Outer volume suppression (OVS) combined with respiratory-triggered point-resolved spectroscopy (PRESS) sequence was used for signal acquisition, with TR = 2 respiratory cycles (~2.0-2.5 s), TE = 15 ms, spectral bandwidth = 3 kHz, 2048 data points, 512 averages, and total scan time of ~15 min. Data Analysis: MR spectra were processed using the MR spectroscopic analysis package provided by manufacturer. The raw data were zero-filled, apodized with a 2-Hz exponential filter, Fourier transformed, 0 th - and 1 st -order phase corrected, and baseline corrected. Major peaks with good SNR were evaluated, which including signal integrals of saturated lipid (-CH3, -CH2-, Sat), unsaturated lipid (-CH=CH-, Unsat) and choline (CCC) at 0.9-1.5, 5.3 and 3.2 ppm, respectively. The area under peak of lipid signals and the choline signal were manually quantified. The Sat and Unsat were measured by normalizing the peak area to CCC. Results In Fig. 1a, the 1 H-MRS spectra of the liver before and after PHx at 12 h and 24 h were shown with Fig. 1b indicates the position of voxel. Peaks of Sat and Unsat lipids had changed at different time points. An increase in Sat and Unsat was observed at 12h and 24 h post-PHx, respectively (Fig. 2). Sat had a maximal increase at 12h (Fig. 2a), and Unsat had a maximal increase at 24h (Fig. 2b). Both lipids decreased at 120h post-PHx. The body weight of the rats decreased by 4% and 7% in average at 12h and 24h post-PHx, respectively, and increased by 4% in average at 120h post-PHx. Discussion In this study, we monitored the change in the three major components in the liver, which included Unsat, CCC and Sat. No significant change was observed in CCC, and total lipids increased. We compared Unsat:CCC with Sat:CCC, as the degree of unsaturation was reported to increase after PHx 3