Improvement of resolution for brain coupled metabolites by optimized ¹H MRS at 7T

Resolution enhancement for glutamate (Glu), glutamine (Gln) and glutathione (GSH) in the human brain by TE-optimized point-resolved spectroscopy (PRESS) at 7T is reported. Sub-TE dependences of the multiplets of Glu, Gln, GSH, γ-aminobutyric acid (GABA) and N-acetylaspartate (NAA) at 2.2-2.6ppm were investigated with density matrix simulations, incorporating three-dimensional volume localization. The numerical simulations indicated that the C4-proton multiplets can be completely separated with (TE₁, TE₂)=(37, 63) ms, as a result of a narrowing of the multiplets and suppression of the NAA 2.5ppm signal. Phantom experiments reproduced the signal yield and lineshape from simulations within experimental errors. In vivo tests of optimized PRESS were conducted on the prefrontal cortex of six healthy volunteers. In spectral fitting by LCModel, Cramér-Rao lower bounds (CRLBs) of Glu, Gln and GSH were 2±1, 5±1 and 6±2 (mean±SD), respectively. To evaluate the performance of the optimized PRESS method under identical experimental conditions, stimulated-echo spectra were acquired with (TE, TM)=(14, 37) and (74, 68) ms. The CRLB of Glu was similar between PRESS and short-TE stimulated-echo acquisition mode (STEAM), but the CRLBs of Gln and GSH were lower in PRESS than in both STEAM acquisitions. Glutamate, glutamine, glutathione and γ-aminobutyric acid in the human brain exhibit severe spectral overlap in short-TE ¹H MRS acquisitions, even at 7 T. The spectral resolution between the C4 proton resonances of the metabolites in the range 2.2-2.6 ppm can be improved with TE optimization. The figure shows a spectrum from the prefrontal cortex, obtained with point-resolved spectroscopy (PRESS) (TE₁, TE₂) = (37, 63) ms at 7.0 T.