In Vivo Glx Measurements From GABA‐Edited HERMES at 3 T Are Not Consistent With Those From Short‐TE PRESS Across Scanners, Brain Regions, Diagnostic and Age Groups

1H-Magnetic resonance spectroscopy (1H-MRS) is a noninvasive technique for quantifying brain metabolites, including glutamate, glutathione (GSH), and γ-aminobutyric acid (GABA), which are essential for brain function and implicated in various neurodevelopmental conditions. As such, 1H-MRS methods that enable reliable and accurate measurement of these metabolites are of considerable clinical value. Hadamard Encoding and Reconstruction of MEGA-Edited Spectroscopy (HERMES; echo time [TE] = 80 ms) is a spectral editing technique that allows for the simultaneous quantification of GABA and GSH, using subtraction approaches to resolve these metabolites in a difference spectrum. Additionally, glutamate plus glutamine resonances (Glx) can be resolved either from the HERMES GABA-edited difference spectrum (GABA-DIFF) or from the sum of all HERMES transients (SUM spectrum). However, the reliability of 80-ms HERMES for quantification of Glx has not been systematically assessed. Here, we evaluate the agreement between Glx obtained from HERMES GABA-DIFF and SUM spectra with Glx derived from short-TE PRESS (TE = 35 ms), which is conventionally used for Glx estimation and has demonstrated reproducibility. Data were acquired from 139 participants across two brain regions (ACC and Thalamus voxels), three scanners, two diagnostic groups (autism and neurotypical development) and two age groups (adolescent/adult and preschooler). Comparisons were made using both creatine-scaled and tissue-corrected Glx estimates. Our findings demonstrate significant systematic and proportional bias between Glx estimates from HERMES (SUM and GABA-DIFF) and short-TE PRESS, consistent across scanners, voxels, age groups and diagnostic categories. These findings indicate that Glx estimates derived from HERMES are not directly comparable to those from short-TE PRESS, and this discrepancy is consistent across a multisite study setting. This underscores the importance of sequence selection and careful methodological consideration when integrating and interpreting data from 1H-MRS across different acquisition protocols.