Resolving Glycerol-CH2 Signal from Water In in vivo 1h Magnetic Resonance Spectroscopy at 9.4 T
Abstract
Purpose
To resolve the glycerol-CH2 resonance (≈4–4.4ppm) from that of water (≈4.7ppm) in 1H magnetic resonance spectroscopy (MRS) at 9.4T.
Methods
A long echo time (TE) approach was used to exploit the shorter T2 relaxation time of water. The glycerol-CH2 signal is J-modulated; therefore, TE values were optimized to retain an in-phase positive or negative glycerol-CH2 signal. A Point RESolved Spectroscopy (PRESS) sequence (TE1=15ms) was employed at 9.4T to acquire safflower oil spectra with a short TE of 25ms and with long TE values ranging from 60-200ms in 10ms increments. The optimal TE values, together with a TE of 25ms, were applied in vivo to a high-fat diet (60% kcal% fat) fed CD1 mouse. Spectra were acquired from visceral adipose tissue and liver to assess the feasibility of the method in different tissues. Liver spectra were also acquired with a CHEmical Shift Selective (CHESS) water suppression module (300Hz pulse bandwidth) and were quantified using LCModel.
Results
Long TE values of 70ms and 170ms were selected, yielding glycerol-CH2 areas of magnitude 41.0% and 7.2% of that of the short TE, respectively. The TE values resolved glycerol-CH2 signal from water in adipose tissue but yielded negligible signal in liver, likely due to shorter hepatic T2 times. However, the relatively narrower water peak in liver rendered water suppression feasible. Short-TE PRESS with water suppression yielded a well-resolved glycerol-CH2 signal. LCModel analysis of the water-suppressed spectrum resulted in a glycerol-CH2 area ≈0.6 times that of the non-suppressed spectrum. The lower signal may be due to less water contamination and some suppression of glycerol signal by the water suppression module.
Conclusion
Long-TE PRESS resolves the glycerol-CH2 resonance in adipose tissue but is unsuitable in liver due to shorter T2 times. Short-TE acquisition with water suppression is more appropriate for its quantification in liver.