• Vivek Tiwari, Zhongxu An, Yiming Wang, and Changho Choi.
• Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, Texas.
• Magn Reson Med. 2018 Oct 1; 80 (4): 1307-1319.

PurposeTo develop 1 H MR spectroscopy that provides distinction of γ-aminobutyric acid (GABA) signal at 3 T in vivo.MethodsTriple-refocusing was tailored at 3 T, with numerical simulations and phantom validation, for distinction of the GABA 2.29-ppm resonance from the neighboring glutamate resonance. The optimization was performed on the inter-RF pulse time delays and the duration and carrier frequency of a non-slice-selective RF pulse. The optimized triple refocusing was tested in multiple regions in 6 healthy subjects, including hippocampus. The in vivo spectra were analyzed with the LCModel using in-house basis spectra. After normalization of the metabolite signal estimates to water, the metabolite concentrations were quantified with reference to medial-occipital creatine at 8 mM.ResultsA triple-refocusing scheme with optimized inter-RF pulse time delays (TE = 74 ms) was obtained for GABA detection. With optimized duration (14 ms) and carrier frequency (4.5 ppm) of the non-slice-selective RF pulse, the triple refocusing gave rise to distinction between the GABA 2.29-ppm and glutamate 2.35-ppm signals. The GABA 2.29-ppm signal was clearly discernible in spectra in vivo (voxel size 4 to 12 mL; scan times 4.3 to 17 minutes). With a total of 24 spectra from 6 gray or white matter-dominant regions, the GABA concentration was measured to be 0.62 to 1.15 mM (Cramer-Rao lower bound of 8 to 14%), and the glutamate level 5.8 to 11.2 mM (Cramer-Rao lower bound of 3 to 6%).ConclusionThe optimized triple refocusing provided distinction between GABA and glutamate signals and permitted direct codetection of these metabolites in the human brain at 3 T in vivo.© 2018 International Society for Magnetic Resonance in Medicine.

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