Elucidation of the downfield spectrum of human brain at 7 T using multiple inversion recovery delays and echo times

Purpose: To characterize the downfield spectrum at 5–10 ppm in the human brain at a high magnetic field of 7 T. Knowledge of relaxation parameters is of interest for spectroscopy as well as chemical exchange–dependent saturation transfer experiments. Methods: Water-suppressed spectra were recorded as echo time and inversion time series in healthy volunteers to investigate T₂ and T₁ values of downfield peaks in gray matter at 7T. The spectra were fitted in a two-dimensional fashion to a heuristic model of a series of Voigt lines, and the relaxation times were obtained for 12 peaks of interest. Results: The mean T₂ values averaged over the volunteers ranged from 24 to 158 ms, whereas the mean T₁ values ranged from 0.22 to 2.40 s. Spectra of specific inversion and echo times revealed superposition of the amide peaks of N-acetylaspartate with short T₂ and an inhomogeneously broadened component with longer T₂. Conclusions: T₂ values were shorter than expected for most peaks, whereas T₁ values had a very wide range; shorter relaxation times for some peaks suggests the presence of macromolecules. Most of the larger peaks seemed to be composed of overlapping components, because the Gaussian widths in the Voigt line shape descriptions were larger than expected based on field inhomogeneities. Magn Reson Med 78:11–19, 2016.