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Abstract:
A method to estimate phosphorus (31 P) transversal relaxation times (T2 ) of coupled spin systems is demonstrated. Additionally, intracellular and extracellular pH (pHext , pHint ) and relaxation corrected metabolite concentrations are reported. Echo time (TE) series of 31 P metabolite spectra were acquired using STEAM localization. Spectra were fitted using LCModel with accurately modeled VeSPA basis sets accounting for J-evolution of the coupled spin systems. T2 s were estimated by fitting a single exponential two-parameter model across the TE series. Fitted inorganic phosphate frequencies were used to calculate pH, and estimated relaxation times were used to determine the relaxation corrected brain metabolite concentrations on an assumption of 3 mM γ-ATP. The method was demonstrated in the healthy human brain at a field strength of 9.4T. T2 relaxation times of ATP and NAD are the shortest between 8 ms and 20 ms, followed by T2 s of inorganic phosphate between 25 ms and 50 ms, and PCr with a T2 of 100 ms. Phosphomonoesters and -diesters have the longest T2 s of about 130 ms. Measured T2 s are comparable to literature values and fit in a decreasing trend with increasing field strengths. Calculated pHs and metabolite concentrations are also comparable to literature values.
