PRESS timings for resolving 13 C 4 -glutamate 1 H signal at 9.4 T: Demonstration in rat with uniformly labelled 13 C-glucose.

MRS of C-13(4)-labelled glutamate (C-13(4)-Glu) during an infusion of a carbon-13 (C-13)-labelled substrate, such as uniformly labelled glucose ([U-C-13(6)]-Glc), provides a measure of Glc metabolism. The presented work provides a single-shot indirect C-13 detection technique to quantify the approximately 2.51 ppm C-13(4)-Glu satellite proton (H-1) peak at 9.4 T. The methodology is an optimized point-resolved spectroscopy (PRESS) sequence that minimizes signal contamination from the strongly coupled protons of N-acetylaspartate (NAA), which resonate at approximately 2.49 ppm. J-coupling evolution of protons was characterized numerically and verified experimentally. A (TE1, TE2) combination of (20 ms, 106 ms) was found to be suitable for minimizing NAA signal in the 2.51 ppm H-1 C-13(4)-Glu spectral region, while retaining the C-13(4)-Glu H-1 satellite peak. The efficacy of the technique was verified on phantom solutions and on two rat brains in vivo during an infusion of [U-C-13(6)]-Glc. LCModel was employed for analysis of the in vivo spectra to quantify the 2.51 ppm H-1 C-13(4)-Glu signal to obtain Glu C4 fractional enrichment time courses during the infusions. Cramer-Rao lower bounds of about 8% were obtained for the 2.51 ppm C-13(4)-Glu H-1 satellite peak with the optimal TE combination.