PRESS timings for resolving ¹³C₄‐glutamate ¹H signal at 9.4 T: Demonstration in rat with uniformly labelled ¹³C‐glucose

Abstract

MRS of 13C4‐labelled glutamate (13C4‐Glu) during an infusion of a carbon‐13 (13C)‐labelled substrate, such as uniformly labelled glucose ([U‐13C6]‐Glc), provides a measure of Glc metabolism. The presented work provides a single‐shot indirect 13C detection technique to quantify the approximately 2.51 ppm 13C4‐Glu satellite proton (1H) 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 1H 13C4‐Glu spectral region, while retaining the 13C4‐Glu 1H satellite peak. The efficacy of the technique was verified on phantom solutions and on two rat brains in vivo during an infusion of [U‐13C6]‐Glc. LCModel was employed for analysis of the in vivo spectra to quantify the 2.51 ppm 1H 13C4‐Glu signal to obtain Glu C4 fractional enrichment time courses during the infusions. Cramér‐Rao lower bounds of about 8% were obtained for the 2.51 ppm 13C4‐Glu 1H satellite peak with the optimal TE combination.