INTRODUCTION

Glutamate (Glu) is the main excitatory neurotransmitter in the central nervous system. The predominantly astroglial glutamine (Gln) is connected to neuronal glutamate via the glutamate-glutamine cycle. Glutathione (GSH) is an antioxidant present in most cell types including astrocytes. Concentration changes in Glu, Gln, and GSH have been reported in several neurological and psychiatric disorders such as epilepsy, schizophrenia, Alzheimer’s disease, and major depressive disorder. Choi et al.¹ previously described that the H4 peaks of Glu, Gln, and GSH are sharp and of high intensity when TE is 100 - 110 ms. At TE =106 ms, it was shown that Glu, Gln, and GSH signals can be maximized while suppressing the N-acetylaspartate (NAA) signals at 2.5 ppm using an editing pulse applied at 4.38 ppm². More recently, a spectral editing technique named multi-edit³ was described for simultaneously measuring Glu, Gln, γ-aminobutyric acid (GABA), and GSH at 7 T using a three-step editing approach at TE = 56 ms. The editing RF pulse was set to OFF, ON at 1.89 ppm, and ON at 2.12 ppm. The first two editing steps mainly provided information for measuring GABA and the NAA singlet signals. Most of the Glu, Gln, and GSH signals came from the third editing step where the editing pulse was applied at 2.12 ppm. In this work, we propose to use single-step spectral editing, i.e., the third editing step of multi-edit, to simultaneously measure Glu, Gln, and GSH at 7 T. The purpose of the editing pulse is to induce signal change of the targeted metabolites from multiplets into high intensity pseudo singlets. Here, the editing pulse acts on the H3 protons of Glu, Gln, and GSH near 2.12 ppm to alter the J-evolution of the corresponding H4 protons such that they form sharp and high intensity peaks.

METHODS

The pulse sequence is given in Figure 1. Spectra of Glu, Gln, and GSH were numerically computed using density matrix simulations to compare with the previously described single-step editing technique using TE = 106 ms². In vivo studies were also performed on 8 healthy volunteers using a Siemens Magnetom 7 T scanner. The reconstructed spectra were fitted in the range of 1.8 – 3.7 ppm by a linear combination of numerically computed basis functions of acetate, NAA, N-acetylaspartylglutamate (NAAG), GABA, Glu, Gln, GSH, aspartate, total creatine (tCr), total choline (tCho), myo-inositol (mIns), taurine, scyllo-inositol, and glycine, as well as a cubic spline baseline.

RESULTS

Figure 2 shows that the single-step editing technique with TE = 56 ms yields a slightly lower Glu peak but much higher Gln and GSH peaks compared to the previous technique with TE = 106 ms² when the T₂ relaxation effects of the metabolites are ignored. As shown in Figure 3, when the T₂ relaxation effects of the metabolites are considered, the peak amplitudes of Glu, Gln, and GSH by editing at TE = 56 ms become significantly higher than at TE = 106 ms. The peak amplitude ratios between TE = 56 ms and 106 ms were 1.09 for Glu, 1.92 for Gln, and 1.79 for GSH. Because the reported in vivo T₂ values of Gln and GSH were not reliable due to their low concentrations, their T₂ values were set to be the same as that of Glu, which was reported to be 184 ms⁴. This T₂ is likely longer than the real T₂ values of Gln and GSH because Gln is an electrically neutral molecule and the T₁ of GSH is unusually short^{4, 5}. Therefore, the true in vivo signal enhancement due to the use of a shorter TE of 56 ms is likely larger than the ratios given by these numerical calculations.

The reconstructed spectra of two MRS scans from one subject are displayed in Figure 4. We can see that the fitted Gln and GSH peaks are both sharp and of high intensity, which is highly desirable for quantification. The two spectra are highly consistent, demonstrating excellent reproducibility. Metabolite concentrations (/[tCr]) measured from 8 healthy volunteers are given in Table 1, which have small CV values and agree well with previous studies^1-3.

DISCUSSION AND CONCLUSION

An editing pulse acting on the H3 protons of Glu, Gln, and GSH near 2.12 ppm was used to make the H4 protons of these metabolites form sharp and high intensity pseudo singlets at a medium TE of 56 ms in a single-step editing experiment. Numerical simulations showed that this single-step spectral editing sequence with TE = 56 ms increased the peak amplitude by 92% for Gln and 79% for GSH compared to a previously described technique using TE = 106 ms. In vivo scans from healthy volunteers yielded high quality spectra with high reproducibility. Because the H4 protons of Glu and Gln can be used to indirectly measure ¹³C labeling of glutamate and glutamine at the spatial resolution and sensitivity of proton MRS, the nearly complete separation between Glu and Gln H4 pseudo singlets and the large enhancement of the much weaker Gln H4 signals at TE = 56 ms described here are expected to facilitate proton detection of glutamate and glutamine metabolism and neurotransmission after introducing exogenous ¹³C-labels.

Acknowledgements

No acknowledgement found.