T2 relaxation times of macromolecules and metabolites in the human brain at 9.4 T

Purpose Relaxation times can contribute to spectral assignment. In this study, effective T-2 relaxation times (T2eff) of macromolecules are reported for gray and white matter-rich voxels in the human brain at 9.4 T. The T2eff of macromolecules are helpful to understand their behavior and the effect they have on metabolite quantification. Additionally, for absolute quantification of metabolites with magnetic resonance spectroscopy, appropriate T-2 values of metabolites must be considered. The T-2 relaxation times of metabolites are calculated after accounting for TE/sequence-specific macromolecular baselines. Methods Macromolecular and metabolite spectra for a series of TEs were acquired at 9.4 T using double inversion-recovery metabolite-cycled semi-LASER and metabolite-cycled semi-LASER, respectively. The T-2 relaxation times were calculated by fitting the LCModel relative amplitudes of macromolecular peaks and metabolites to a mono-exponential decay across the TE series. Furthermore, absolute concentrations of metabolites were calculated using the estimated relaxation times and internal water as reference. Results The T2eff of macromolecules are reported, which range from 13 ms to 40 ms, whereas, for metabolites, they range from 40 ms to 110 ms. Both macromolecular and metabolite T-2 relaxation times are observed to follow the decreasing trend, with increasing B-0. The linewidths of metabolite singlets can be fully attributed to T-2 and B-0 components. However, in addition to these components, macromolecule linewidths have contributions from J-coupling and overlapping resonances. Conclusion The T-2 relaxation times of all macromolecular and metabolite peaks at 9.4 T in vivo are reported for the first time. Metabolite relaxation times were used to calculate the absolute metabolite concentrations.