Long TE PRESS and STEAM for measuring the triglyceride glycerol CH₂ protons at 3 T

Abstract

The glycerol methylene proton resonances (4–4.5 parts per million, ppm), which arise from the triglyceride backbone, are relevant to fat composition assessment and can be measured with proton MRS. The purpose of the presented work is to determine long TE (echo time) point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM) values at 3 T to resolve the glycerol resonances from that of overlapping water. The response of the glycerol methylene protons of nine edible oils as a function of PRESS and STEAM TE (mixing time, TM = 20 ms) was investigated. In addition, high resolution NMR spectra of the oils were acquired at 16.5 T. Long TE values where J‐coupling losses were lowest were selected, namely a TE of 180 ms for PRESS (first echo time 17 ms) and a TE of 100 ms for STEAM (mixing time 20 ms). Oil olefinic (≈5.4 ppm) to glycerol ratios were calculated from the long TE spectra and correlated with 16.5 T ratios. The two techniques yielded olefinic/glycerol ratios that correlated with 16.5 T ratios (R2 = 0.79 for PRESS and 0.90 for STEAM). The efficacy of the sequences in resolving the glycerol resonance from that of water was verified in vivo on tibial bone marrow of four healthy volunteers. In addition, the potential for using the glycerol methylene signal normalized to the methyl signal (≈0.9 ppm) to assess changes in free fatty acid content was demonstrated by measuring differences in spectra acquired from a triglyceride peanut oil phantom and from a phantom composed of a mixture of peanut oil and free fatty acid oleic acid.

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

“…The long TE of 100 ms is more applicable to in vivo situations where water is present; the longer TE resolves the glycerol signal from water signal due to the shorter T 2 of water. 10 The larger percent differences in ratios for the longer TE are likely due to accentuation of differences in J-coupling evolution and T 2 relaxation of the diglyceride and triglyceride glycerol protons. However, the long TE also offers the benefit of less signal contamination from the triglyceride protons in the diglyceride spectral region of 3.85-4.2 ppm.…”

“…Fat in the body is primarily found in the form of triglycerides, where a fatty acid is attached to each of the three carbons in a glycerol backbone 7 . The response (due to J ‐coupling evolution) of triglyceride glycerol protons in the spectral range of 4–4.5 ppm has been previously characterized in vivo as a function of point resolved spectroscopy (PRESS) 8 and stimulated echo acquisition mode (STEAM) 9 echo time ( TE ) at 3 T 10 . It was shown that STEAM with a TE of 100 ms retained adequate signal from the glycerol resonances and resolved them from that of water by exploiting the shorter T 2 of water 10 .…”

“…The response (due to J ‐coupling evolution) of triglyceride glycerol protons in the spectral range of 4–4.5 ppm has been previously characterized in vivo as a function of point resolved spectroscopy (PRESS) 8 and stimulated echo acquisition mode (STEAM) 9 echo time ( TE ) at 3 T 10 . It was shown that STEAM with a TE of 100 ms retained adequate signal from the glycerol resonances and resolved them from that of water by exploiting the shorter T 2 of water 10 . The efficacy of using the glycerol resonances as a reference signal was verified in tibial bone marrow in vivo; in addition, the feasibility of using the glycerol resonances to estimate relative levels of free fatty acid to triglyceride content was demonstrated in vitro 10 .…”

“…10 The efficacy of using the glycerol resonances as a reference signal was verified in tibial bone marrow in vivo; in addition, the feasibility of using the glycerol resonances to estimate relative levels of free fatty acid to triglyceride content was demonstrated in vitro. 10 Just as free fatty acid content can be altered in disease, 11 it has been reported that diglyceride (only two fatty acids attached to the glycerol backbone) levels are elevated in liver disease [12][13][14] and are associated with insulin resistance. 15 Higher amounts of diglycerides have also been found with elevated levels of adrenocorticotropic hormone (ACTH).…”

“…(due to J-coupling evolution) of triglyceride glycerol protons in the spectral range of 4-4.5 ppm has been previously characterized in vivo as a function of point resolved spectroscopy (PRESS) 8 and stimulated echo acquisition mode (STEAM) 9 echo time (TE) at 3 T. 10 It was shown that STEAM with a TE of 100 ms retained adequate signal from the glycerol resonances and resolved them from that of water by exploiting the shorter T 2 of water. 10 The efficacy of using the glycerol resonances as a reference signal was verified in tibial bone marrow in vivo; in addition, the feasibility of using the glycerol resonances to estimate relative levels of free fatty acid to triglyceride content was demonstrated in vitro. 10 Just as free fatty acid content can be altered in disease, 11 it has been reported that diglyceride (only two fatty acids attached to the glycerol backbone) levels are elevated in liver disease [12][13][14] and are associated with insulin resistance.…”