MR imaging of the amide-proton transfer effect and the pH-insensitive nuclear overhauser effect at 9.4 T

The amide proton transfer (APT) effect has emerged as a unique endogenous molecular imaging contrast mechanism with great clinical potentials. However, in vivo quantitative mapping of APT using the conventional asymmetry analysis is difficult due to the confounding nuclear Overhauser effect (NOE) and the asymmetry of the magnetization transfer effect. Here, we showed that the asymmetry of magnetization transfer contrast from immobile macromolecules is highly significant, and the wide spectral separation associated with a high magnetic field of 9.4 T delineates APT and NOE peaks in a Z-spectrum. Therefore, high-resolution apparent APT and NOE maps can be obtained from measurements at three offsets. The apparent APT value was greater in gray matter compared to white matter in normal rat brain and was sensitive to tissue acidosis and correlated well with apparent diffusion coefficient in the rat focal ischemic brain. In contrast, no ischemia-induced contrast was observed in the apparent NOE map. The concentration dependence and the pH insensitivity of NOE were confirmed in phantom experiments. Our results demonstrate that in vivo apparent APT and NOE maps can be easily obtained at high magnetic fields and the pH-insensitive NOE may be a useful indicator of mobile macromolecular contents.