The proton T2 relaxation times of metabolites in the human brain were measured using point-resolved spectroscopy at 3T in vivo. Four echo times (54, 112, 246 and 374 ms) were selected from numerical and phantom analyses for effective detection of the glutamate multiplet at ~2.35 ppm. In vivo data were obtained from medial occipital and left occipital cortices of five healthy volunteers, which contained predominantly gray and white matter, respectively. Spectra were analyzed with LCModel software using volume-localized calculated spectra of brain metabolites. The estimate of the signal strength vs. TE was fitted to a monoexponential function for estimation of apparent T2 (T2†). The T2† was estimated to be similar between the brain regions for creatine, choline, glutamate and myo-inositol, but significantly different for the N-acetylaspartate singlet and multiplet. The T2†s of glutamate and myo-inositol were measured as 181±16 and 197±14 ms (mean±SD, N = 5) for medial occipital, and 180±12 and 196±17 ms for left occipital, respectively.
A membrane with interpenetrating networks between poly͑vinyl alcohol͒ ͑PVA͒ and poly͑styrene sulfonic acid͒ ͑PSSA͒ coupled with a high proton conductivity is realized and evaluated as a proton exchange membrane electrolyte for a direct methanol fuel cell ͑DMFC͒. Its reduced methanol permeability and improved performance in DMFCs suggest the new blend as an alternative membrane to Nafion membranes. The membrane has been characterized by powder X-ray diffraction, scanning electron microscopy, time-modulated differential scanning calorimetry, and thermogravimetric analysis in conjunction with its mechanical strength. The maximum proton conductivity of 3.3 ϫ 10 −2 S/cm for the PVA-PSSA blend membrane is observed at 373 K. From nuclear magnetic resonance imaging and volume localized spectroscopy experiments, the PVA-PSSA membrane has been found to exhibit a promising methanol impermeability, in DMFCs. On evaluating its utility in a DMFC, it has been found that a peak power density of 90 mW/cm 2 at a load current density of 320 mA/cm 2 is achieved with the PVA-PSSA membrane compared to a peak power density of 75 mW/cm 2 at a load current density of 250 mA/cm 2 achievable for a DMFC employing Nafion membrane electrolyte while operating under identical conditions; this is attributed primarily to the methanol crossover mitigating property of the PVA-PSSA membrane. Direct methanol fuel cells ͑DMFCs͒ using a proton exchange membrane have been identified as one of the most promising candidates for portable power applications.1,2 Unlike hydrogen-air polymer electrolyte fuel cells, DMFCs do not require a fuel reformer or a high-volume hydrogen storage system. The membrane electrolyte employed with the DMFC, besides exhibiting a good proton conductivity, should act as a physical separator to prevent fuel crossover from the anode to the cathode. At present, Nafion a perfluorosulfonated membrane with a hydrophobic fluorocarbon backbone and hydrophilic sulfonic acid pendant side chains, happens to be the only commercially available and widely used membrane electrolyte in the DMFC. It has been documented that proton conduction in Nafion occurs through the ionic channels formed by micro-or nanophase separation between the hydrophilic proton exchange sites and the hydrophobic domains.3 However, the methanol crossover from anode to cathode across the Nafion membrane brings about a mixed potential at the cathode causing both the loss of fuel and cell polarization impeding their commercial realization. [4][5][6] It has been reported that even over 40% of methanol could be lost in a DMFC due to crossover across the membrane.7 Methanol crossover across the Nafion membrane can be kept to a minimum by controlling the methanol-feed concentration. Alternatively, membranes that are relatively impermeable to methanol have been employed for this purpose. [8][9][10][11][12] Membranes with a lower methanol permeability allow a higher methanol-feed concentration, enhancing the performance of the DMFC. To optimize fuel cell performance, it is neces...
The 1H resonances of GABA in the human brain in vivo are extensively overlapped with the neighboring abundant resonances of other metabolites and remain indiscernible in short TE MRS at 7T. Here we report the GABA resonance at 2.28 ppm can be fully resolved by means of echo time optimization of a point-resolved spectroscopy (PRESS) scheme. Following numerical simulations and phantom validation, the subecho times of PRESS was optimized at (TE1, TE2) = (31, 61) ms for detection of GABA, glutamate (Glu), glutamine (Gln) and glutathione (GSH). The in-vivo feasibility of the method was tested in several brain regions in 9 healthy subjects. Spectra were acquired from the medial prefrontal, left frontal, medial occipital and left occipital brain and analyzed with LCModel. Following the gray and white matter (GM and WM) segmentation of T1-weighted images, linear regression of metabolite estimates was performed against the fractional GM contents. The GABA concentration was estimated to be about 7 fold higher in GM than in WM. GABA was overall higher in frontal than in occipital brain. Glu was ~2 fold higher in GM than in WM in both frontal and occipital brain. Gln was significantly different between frontal GM and WM while being similar between occipital GM and WM. GSH did not show significant dependence on tissue content. The signals from N-acetylaspartylglutamate were clearly resolved, giving the concentration higher by > 10 fold in WM than in GM. Our data indicate that the PRESS TE = 92 ms method provides an effective means for measuring GABA and several challenging J-coupled spin metabolites in human brain at 7T.
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