Author: José Antonio Klautau Toffoli, Anthony G. Tessier, Atiyah Yahya 👨🔬
Affiliation: Department of Oncology, University of Alberta 🌍
Purpose: To demonstrate that magnetic resonance spectroscopy (MRS) methyl (≈0.9 ppm) to glycerol (≈4.1-4.3 ppm) resonance area ratios correlate with relative concentrations of free fatty acids to triglycerides.
Methods: Experiments were conducted with an animal 9.4 T magnetic resonance scanner using a volume coil. A Point RESolved Spectroscopy (PRESS) sequence with a total echo time (TE) of 21 ms (TE1/TE2=11.8/9.2) was employed. Spectra were obtained from purchased custom made phantoms containing varying amounts of free fatty acids (oleic acid) and triglycerides (triolein) by weight: oleic acid/triolein weight ratios of 50%/50%, 35%/65%, 25%/75%, 15%/85%, and 5%/95%. Spectra were acquired from 5x5x5 mm3 voxels (32 averages with a repetition time of 5 seconds). To assess reproducibility, five spectra were acquired per phantom. Direct integration was performed to obtain methyl (0.8–1.0 ppm) and glycerol CH2 (4.0-4.4 ppm) resonance areas and the methyl to glycerol ratio was calculated for each spectrum. Mean values and standard deviations of the methyl to glycerol ratios were computed for each phantom over the five spectra. Values were plotted against free oleic acid to triolein concentration ratios for each phantom (determined from the weight ratios using the molecular weights). The correlation was evaluated using a weighted least squares regression.
Results: Glycerol to methyl area ratios linearly correlate with free oleic acid to triolein concentration ratios (R2w =1.0). The linear relationship results from methyl protons being present in both triglycerides (9 protons) and free fatty acids (3 protons) and glycerol protons only being in triglycerides (4 protons), thereby, yielding a methyl to glycerol ratio given by (9 [triglyceride] + 3 [free fatty acid])/(4 [triglyceride]).
Conclusion: MRS methyl to glycerol CH2 resonance area ratios enable quantification of relative free fatty acid to triglyceride concentrations in phantoms at 9.4 T. Future work will investigate the efficacy of the method in vivo.