Estimating relative diglyceride to triglyceride content with localized MRS at 3 T

It has been previously shown that the MRS sequence stimulated echo acquisition mode (STEAM; mixing time, TM  = 20 ms) with an echo time (TE) of 100 ms resolves triglyceride glycerol resonances from that of water at 3 T. The purpose of this work is to determine if STEAM with a TE of 100 ms facilitates relative quantification of diglyceride/triglyceride levels at 3 T. Spectra were obtained from tricaprylin (triglyceride) and dicaprylin (diglyceride) with a range of STEAM TE values (TM = 20 ms). TE values that resulted in two resolved glycerol resonances for triglycerides (rendering them suitable for distinguishing triglyceride contributions from those of diglycerides) were selected. One resonance resides in the 3.85-4.2 ppm spectral range (overlapping the 1,3-diglyceride resonance) and the other in the 4.2-4.6 ppm spectral range (overlapping one of the 1,2-diglyceride resonances). STEAM with TE values of 40 ms and 100 ms (TM = 20 ms) yielded two resolvable triglyceride resonances (tricaprylin phantom), at about 4 ppm and 4.4 ppm. Direct integration of the resonances showed that the former peak has 0.86 and 0.17 times the area of the latter for TE = 40 ms and 100 ms, respectively. Spectra obtained from the phantoms containing mixtures of diglyceride (1,3-dicaprylin) and triglyceride (tricaprylin) were acquired. The triglyceride contribution to the 4 ppm glycerol resonance, a mixture of signal from 1,3-diglyceride and triglyceride, can be approximated from the area of the 4.4 ppm peak, resulting in an estimate of the 1,3-diglyceride contribution. Analysis was performed for STEAM TE = 40 ms and TE = 100 ms spectra acquired from phantoms with 1,3-dicaprylin/tricaprylin weight/weight contents of 2.5%/97.5%, 5%/95%, 10%/90% and 20%/80%. Concentration ratios of 1,3-dicaprylin/tricaprylin estimated with both STEAM TE values resulted in linear correlations with expected concentration ratios (R²  > 0.99).