Phosphorus transversal relaxation times and metabolite concentrations in the human brain at 9.4 T

A method to estimate phosphorus ( 31 P) transversal relaxation times (T 2 s) of coupled spin systems is demonstrated. Additionally, intracellular and extracellular pH and relaxation‐corrected metabolite concentrations are reported. Echo time (TE) series of 31 P metabolite spectra were acquired using stimulated echo acquisition mode (STEAM) localization. Spectra were fitted using LCModel with accurately modeled Versatile Simulation, Pulses and Analysis (VeSPA) basis sets accounting for J‐evolution of the coupled spin systems. T 2 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 healthy human brain at a field strength of 9.4 T. T 2 times of ATP and nicotinamide adenine dinucleotide (NAD) were shortest between 8 and 20 ms, followed by T 2 s of inorganic phosphate between 25 and 50 ms, and phosphocreatine with a T 2 of 100 ms. Phosphomonoesters and phosphodiesters had the longest T 2 s of about 130 ms. The measured T 2 s are comparable with literature values and fit in a decreasing trend with increasing field strengths. Calculated pHs and metabolite concentrations are also comparable with literature values.