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

PurposeTo characterize the downfield spectrum at 5-10ppm in the human brain at a high magnetic field of 7T. Knowledge of relaxation parameters is of interest for spectroscopy as well as chemical exchange-dependent saturation transfer experiments. MethodsWater-suppressed spectra were recorded as echo time and inversion time series in healthy volunteers to investigate T-2 and T-1 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. ResultsThe mean T-2 values averaged over the volunteers ranged from 24 to 158 ms, whereas the mean T-1 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 T-2 and an inhomogeneously broadened component with longer T-2. ConclusionsT(2) values were shorter than expected for most peaks, whereas T-1 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. (c) 2017 International Society for Magnetic Resonance in Medicine