Development and Preclinical Study of Free Radical Imaging Using Field-Cycling Dynamic Nuclear Polarization MRI

Free radicals, such as metabolic intermediates, reactive oxygen species, and metal enzymes, are key substances in organisms, although they can also cause various oxidative diseases. Thus, in vivo free radical imaging should be considered as the ultimate form of metabolic imaging. Unfortunately, electron spin resonance (ESR) imaging has inherent disadvantages, such as free radicals with large linewidths generating blurred images and the presence of two or more free radicals resulting in a complicated imaging procedure. Dynamic nuclear polarization–magnetic resonance imaging (DNP-MRI) is a noninvasive imaging method to visualize in vivo free radicals, theoretically, with the same resolution as the MRI anatomical resolution, and fixed low-field DNP-MRI provides unique information on oxidative diseases and cancer. However, the large gyromagnetic ratio of the electron spin, which is 660-fold greater than that of a proton, requires field cycling, wherein the external magnetic field should be varied during DNP-MRI observations. This causes difficulties in developing a DNP-MRI system for clinical purposes. We developed a novel field-cycling DNP-MRI system for a preclinical study. In the said system, the magnetic field is switched by rotationally moving two magnets, with a magnetic flux density of 0.3 T for MRI and 5 mT for ESR. The image quality was examined using various pulse sequences and ESR irradiation using nitroxyl radical as the phantom, and the optimum conditions were established. Using the system, we performed a preclinical study involving free radical imaging by placing the free radicals under the palm of a human hand.