<h3>Purpose/Objective(s)</h3> Visualization of radiation dose and tracks enable even safer and more accurate radiation therapy. It is already known that free radicals are generated by irradiation and imaging these free radicals will lead to an accurate understanding of the treatment dose and area after radiation therapy. We have been developing the free radical imaging methods using Dynamic Nuclear Polarization (DNP)-MRI with nitroxyl radicals as a redox probe (e.g., 1-Oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine (Tempol)). We previously demonstrated that the mixed solution of Tempol and glutathione (GSH) was useful as a probe for detecting free-radical reactions, allowing us to observe the redox reaction after irradiation clearly. However, one of the problems was that the stability of the solution was limited. In order to apply this technology in vivo, we considered it was necessary to improve its stability. In this study, we focused on 4-Methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo methacrylate; TempoMC) as a new redox probe that is stable in GSH solution, and further improved the imaging method. <h3>Materials/Methods</h3> To confirm the stability of TempoMC in various concentrations of GSH solutions, EPR signals were measured immediately after mixing and after 1 day, 3 days, and 7 days. The reduction of TempoMC by ascorbic acid (AsA) was monitored by X-band EPR spectroscopy to demonstrate the redox imaging capability. To confirm the possibility of redox reaction between TempoMC/GSH solution with free radicals induced by X-ray. The glass capillaries including TempoMC and GSH were irradiated 10 Gy by Linac, and the ESR signals were measured. Free radical imaging by low filed type of dynamic nuclear polarization (DNP)-MRI was performed using phantom filled with TempoMC and GSH solution. <h3>Results</h3> In contrast to Tempol, which showed a sharp EPR signal decrease depending on the concentration of GSH, the signal of TempoMC was generally constant. In addition, TempoMC showed only a slight decrease in signal even after 7 days, proving that it exists in a stable state. After mixing the AsA solution, the ESR signal was gradually decreased by the redox reaction. The EPR signals of TempoMC capillaries were decreased by irradiation. The phantom including TempoMC radical were clearly visualized by DNP-MRI. As described above, TempoMC/GSH solution behaved similar reactivity to Tempol/GSH solution, proving to be more stable and more useful as a redox probe. <h3>Conclusion</h3> We evaluated TempoMC/GSH solution as a new useful redox probe. TempoMC /GSH solution is much more stable than the conventional Tempol/GSH solution, and is expected to be applied in the future, including in vivo.