Glutamate measurements using edited MRS
Muhammad G. Saleh, Andrew Prescot, Linda Chang, Christine Cloak, Eric Cunningham, Punitha Subramaniam, Perry F. Renshaw, Deborah Yurgelun-Todd, Helge J. Zöllner, Timothy P. L. Roberts, Richard A. E. Edden, Thomas Ernst
Abstract
Purpose
To demonstrate J-difference coediting of glutamate using Hadamard encoding and reconstruction of Mescher-Garwood-edited spectroscopy (HERMES).
Methods
Density-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.
Results
HERMES 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.
Conclusion
HERMES 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.