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

Abstract

Resolution enhancement for glutamate (Glu), glutamine (Gln) and glutathione (GSH) in the human brain by TE-optimized point-resolved spectroscopy (PRESS) at 7 T is reported. Sub-TE dependences of the multiplets of Glu, Gln, GSH, γ-aminobutyric acid (GABA) and N-acetylaspartate (NAA) at 2.2-2.6 ppm 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 (TE(1), TE(2)) = (37, 63) ms, as a result of a narrowing of the multiplets and suppression of the NAA 2.5 ppm 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.

Spectral fitting using basis set modified by measuredB0field distribution

“…Previous published metabolite values were calculated as metabolite to Cr concentration ratios and compared with our quantification results. Our quantification results were generally within the range of previously published values (1-3, 7-9, 20, 21). …”

Comparing GABA-dependent physiological measures of inhibition with proton magnetic resonance spectroscopy measurement of GABA using ultra-high-field MRI

“…Detection of some neurometabolites, such 1H MRS can be challenging due to low concentration and signal overlap with more concentrated metabolites. Ultra-high field offers the advantages of increased spectral resolution and signal-to-noise ratio (SNR) compared to lower field strengths (Choi et al, 2010, Stephenson et al, 2011, Tkáč et al, 2001, Tkáč et al, 2009). In the context of the current study it is of interest to note that MRS studies of GABA concentration within the cortical motor areas in TS have reported either no differences in GABA concentration (Draper et al, 2014, Puts et al, 2015, Tinaz et al, 2014) or increased levels of GABA relative to age-matched controls (Draper et al, 2014).…”

Strategies for rapid in vivo 1H and hyperpolarized 13C MR spectroscopic imaging

“…Other options that applied high bandwidth pulses for volume localization included PRESS with spectral spectral pulses and TE = 80 or 35ms (58), interleaved narrow band PRESS with TE=90ms (8), slice select with semi-LASER using a TE of 50ms (10) or 19ms (59). For these longer TE acquisitions, optimizing the sub-echo time sets for point-resolved spectroscopy (PRESS) can also provide separation of glutamate, glutamine, glutathione and γ-aminobutyric acid without the need for sophisticated editing sequences (60). Increased interest in studying these metabolites for assessing abnormalities in cognitive function and for evaluating new treatment paradigms has placed a greater emphasis on developing robust tools for obtaining quantitative estimates of differences between control and patient populations.…”