MEGA-PRESS, a spectral editing method, has gained popularity in the MRS community thanks to its ability to edit low-concentration metabolites with relative ease of implementation, allowing direct and unambiguous measurements of the inhibitory neurotransmitter GABA, the antioxidant glutathione (GSH), and the anaerobic product lactate (Lac). However, current implementations of MEGA-PRESS are diverse across vendors, differing in terms of RF pulse shapes and pulse sequence timings. Recent multi-site data revealed that ~30% of the variance in the GABA+ data is attributed to the site- and vendor-level differences in the implementation of MEGA-PRESS.
Recently, Muhammad Saleh and colleagues developed a new universal MEGA-PRESS sequence for the major MR vendors (Philips, Siemens, GE, and Canon) with common RF pulse shapes and timings, and has functionality for HERMES editing of GABA and GSH (Figure 1). Upon comparing with the existing vendor-native sequences, the universal sequence yielded edited spectra with strong correlations and low variance among vendors at both short and long TEs phantom and in vivo experiments (Figures 2 and 3). The universal sequence includes simultaneous editing of GABA and GSH with HERMES, allowing excellent separation of the edited signals in half the scan time compared with the sequentially acquired conventional MEGA editing. The universal sequence is available collaboratively to the community for application in future studies.
Exciting times ahead: Building on the universal sequence, Georg Oeltzschner and colleagues developed HERCULES sequence, capable of editing seven coupled metabolites that would usually require a single 11-min editing experiment each. This sequence is also available collaboratively to the community for application in future studies.
For more updates, stay tuned for any new developments from Edden's lab. Universal edited MRS sequences will be presented at the 27th Annual Meeting of the ISMRM, Montreal, Canada, 2019. Looking forward to meeting you all.
Figure 1: Pulse sequence diagrams indicating RF pulse shapes and timings for the vendor-native Philips, Siemens, GE and Canon sequences, and the universal sequence at TE = 68 ms.
Figure 2: a) MEGA-PRESS experiment using the GABA phantom (TE = 68 ms) and Lac phantom (TE = 140 ms) from vendor-native sequences (left) and the universal sequence (right). b) Edited spectra from HERMES experiments acquired using the universal sequence, performed in a GABA phantom (left) and a GSH phantom (right).
Figure 4: In vivo experiments using the universal sequence. Spectra acquired on Philips and Siemens scanners are overlaid for each subject: a) MEGA-PRESS GABA (TE = 68 ms) and Lac (TE = 140 ms) spectra; b) GABA- and GSH-edited HERMES spectra. In vivo experiments using the universal sequence for GE (green) and Canon (red) scanners: c) MEGA-PRESS GABA-edited (TE = 68 ms) and Lac-edited (TE = 140 ms) spectra; d) GABA- and GSH-edited HERMES spectra. Spectra acquired are overlaid on the ± 1SD range (in gray) of the amalgamated Philips and Siemens data (6 subjects).