Simultaneous Detection of Glutathione and Lactate using Spectral Editing at 3T
Peter Barker, Johns Hopkins University
Introduction: In brain MR spectroscopy (MR), spectral editing is often used to selectively detect signals from lower concentration metabolites (such as GABA, or glutathione (GSH)) that overlap with those of other, more abundant compounds. Usually, one compound is observed at a time, however there have been examples of editing sequences designed to simultaneously detect 2 compounds at the same time (1,2). This abstract describes the optimization of the ‘MEGA-PRESS’ editing sequence to simultaneously detect GSH and lactate (Lac) in a single acquisition.
Methods: Experiments and numerical simulations were performed to determine the optimum TE and editing pulse frequencies for simultaneous GSH and Lac editing. Spectra were acquired on a Philips ‘Achieva’ 3T scanner using a 32-channel receive head coil. The PRESS sequence was used with frequency-modulated slice selective refocusing pulses (‘fm_ref07’, 2.2 kHz bandwidth) in order to minimize chemical shift displacement effects. Phantom spectra were collected in a phantom containing 50 mM GSH and 25 mM sodium lactate in phosphate-buffered saline (pH = 7.2). In the PRESS experiments, TE was varied from 70 to 280 ms in 10 ms increments. In the MEGA-PRESS experiments using TE 140 ms, the ‘on’ editing pulse frequency was varied from 4.1 to 4.56 ppm in increments of 0.05 ppm, with the ‘off’ pulse placed at 10 ppm. The editing pulse duration was 20 ms (75 Hz bandwidth). Finally, a MEGA-PRESS experiment was performed with an ‘on’ editing pulse at 4.35 ppm with 10 ms duration (150 Hz bandwidth).
Simulations of the MEGA-PRESS experiment for the GSH spin system were performed using MATLAB, including spatially-resolved modulation effects, using the chemical shifts and coupling constants given by Govinderaju et al. (3).
Results: In the phantom, the optimum TE for editing (i.e. most negative signal in the ‘off’ scan) for both GSH and Lac was in the range of 140-150 ms, which was in good agreement with the simulations (Figure 1A). As expected, maximum GSH signal was obtained with an editing pulse frequency of 4.56 ppm, while maximum Lac was observed at 4.10 ppm. By increasing the editing pulse bandwidth (to 150 Hz), and placing it symmetrically between the coupled resonances of Lac and GSH (4.35 ppm), it was possible to edit both resonances simultaneously with ~90% sensitivity of the individual measurements (FIgure 1B).
Discussion: Simultaneous editing of Lac and GSH is possible at 3T. Compared to sequential measurements, simultaneous editing results in a 50% reduction in the scan time while retaining nearly the same sensitivity. Note that the optimization performed here did not consider T2 relaxation effects in vivo, which will tend to shorten the optimal TE. The simultaneous measurement of Lac, an indicator of non-oxidative glycolysis, and GSH (a major anti-oxidant) may be of interest in various brain pathologies, including schizophrenia.
References
1. Terpstra et al., MRM 56:1192-9 (2006)
2. Snoussi et al., ISMRM (2015)
3. Govindaraju et al., NMR Biomed 13:129–53 (2000)