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

Abstract

Purpose: Relaxation times can contribute to spectral assignment. In this study, effective T₂ relaxation times ( $T_2^{\mathrm{eff}}$ ) of macromolecules are reported for gray and white matter-rich voxels in the human brain at 9.4 T. The $T_2^{\mathrm{eff}}$ 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 T₂ values of metabolites must be considered. The T₂ 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 T₂ 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 $T_2^{\mathrm{eff}}$ 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 T₂ relaxation times are observed to follow the decreasing trend, with increasing B₀ . The linewidths of metabolite singlets can be fully attributed to T₂ and B₀ components. However, in addition to these components, macromolecule linewidths have contributions from J-coupling and overlapping resonances.

Conclusion: The T₂ 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.

Keywords: MR spectroscopy; T2 relaxation time; absolute quantification; macromolecules; ultrahigh magnetic field.