Glutamate measurements using edited MRS

Abstract

AbstractPurposeTo demonstrate J‐difference coediting of glutamate using Hadamard encoding and reconstruction of Mescher‐Garwood‐edited spectroscopy (HERMES).MethodsDensity‐matrix simulations of HERMES (TE 80 ms) and 1D J‐resolved (TE 31–229 ms) of glutamate (Glu), glutamine (Gln), γ‐aminobutyric acid (GABA), and glutathione (GSH) were performed. HERMES comprised four sub‐experiments with editing pulses applied as follows: (A) 1.9/4.56 ppm simultaneously (ONGABA/ONGSH); (B) 1.9 ppm only (ONGABA/OFFGSH); (C) 4.56 ppm only (OFFGABA/ONGSH); and (D) 7.5 ppm (OFFGABA/OFFGSH). Phantom HERMES and 1D J‐resolved experiments of Glu were performed. Finally, in vivo HERMES (20‐ms editing pulses) and 1D J‐resolved (TE 31–229 ms) experiments were performed on 137 participants using 3 T MRI scanners. LCModel was used for quantification.ResultsHERMES simulation and phantom experiments show a Glu‐edited signal at 2.34 ppm in the Hadamard sum combination A+B+C+D with no overlapping Gln signal. The J‐resolved simulations and phantom experiments show substantial TE modulation of the Glu and Gln signals across the TEs, whose average yields a well‐resolved Glu signal closely matching the Glu‐edited signal from the HERMES sum spectrum. In vivo quantification of Glu show that the two methods are highly correlated (p < 0.001) with a bias of ∼10%, along with similar between‐subject coefficients of variation (HERMES/TE‐averaged: ∼7.3%/∼6.9%). Other Hadamard combinations produce the expected GABA‐edited (A+B–C–D) or GSH‐edited (A–B+C–D) signal.ConclusionHERMES simulation and phantom experiments show the separation of Glu from Gln. In vivo HERMES experiments yield Glu (without Gln), GABA, and GSH in a single MRS scan.