Quantitative characterization of nuclear overhauser enhancement and amide proton transfer effects in the human brain at 7 Tesla

Purpose This study aimed to quantitatively investigate two main magnetization transfer effects at low B₁: the nuclear Overhauser enhancement (NOE) and amide proton transfer in the human brain at 7 T. Methods The magnetization transfer effects in the human brain were characterized using a four-pool proton model, which consisted of bulk water, macromolecules, an amide group of mobile proteins and peptides, and NOE-related protons resonating upfield. The pool sizes, exchange rates, and relaxation times of these proton pools were investigated quantitatively by fitting, and the net signals of amide proton transfer and NOE were simulated based on the fitted parameters. Results The results showed that the four-pool model fitted the experimental data quite well, and the NOE effects in human brain at 7 T had a broad spectrum distribution. The NOE effects peaked at a B₁ of ∼ 1-1.4 μT and were significantly stronger in the white matter than in the gray matter, corresponding to a pool-size ratio ∼ 2:1. As the amide proton transfer effect was relatively small compared with the NOE effects, magnetization transfer asymmetry analysis yielded an NOE-dominated contrast in the healthy human brain in this range of B₁. Conclusion These findings are important to identify the source of NOE effects and to quantify amide proton transfer effects in human brain at 7 T.