In vivo characterization of the downfield part of ¹H MR spectra of human brain at 9.4 T: Magnetization exchange with water and relation to conventionally determined metabolite content

Purpose: To perform exchange-rate measurements on the in vivo human brain downfield spectrum (5–10 ppm) at 9.4 T and to compare the variation in concentrations of the downfield resonances and of known upfield metabolites to determine potential peak labels. Methods: Non-water-suppressed metabolite cycling was used in combination with an inversion transfer technique in two brain locations in healthy volunteers to measure the exchange rates and T₁ values of exchanging peaks. Spectra were fitted with a heuristic model of a series of 13 or 14 Voigt lines, and a Bloch–McConnell model was used to fit the exchange rate curves. Concentrations from non-water-inverted spectra upfield and downfield were compared. Results: Mean T₁ values ranged from 0.40 to 0.77 s, and exchange rates from 0.74 to 13.8 s⁻¹. There were no significant correlations between downfield and upfield concentrations, except for N-acetylaspartate, with a correlation coefficient of 0.63 and P < 0.01. Conclusions: Using ultrahigh field allowed improved separation of peaks in the 8.2 to 8.5 ppm amide proton region, and the exchange rates of multiple downfield resonances including the 5.8-ppm peak, previously tentatively assigned to urea, were measured in vivo in human brain. Downfield peaks consisted of overlapping components, and largely missing correlations between upfield and downfield resonances—although not conclusive—indicate limited contributions from metabolites present upfield to the downfield spectrum. Magn Reson Med 79:2863–2873, 2018.