Imaging of Ra-223 solution using an optical method

Ra-223 has recently been introduced in alpha-targeting radionuclide therapy. According to the decay scheme of Ra-223, beta particles with higher energy than the Cerenkov-light threshold are emitted. In addition, there are reports that the alpha particles themselves emit luminescence in water and air. However, measured optical imaging results from Ra-223 have not yet been reported. Therefore, we conducted optical imaging of an Ra-223 solution and compared it with that of an F-18 solution. Ra-223 and F-18 solutions contained in transparent glass vials were imaged using a high-sensitivity CCD camera. We could obtain an image of the Ra-223 solution with radioactivity of 0.72 MBq in an acquisition time of less than 10 s. At the same level of radioactivity, the intensity of the optical signal in the solution part of the vial of Ra-223 was 21 times higher than that of F-18. Furthermore, in the air part of the vials, Ra-223 had higher luminescence than that of F-18. The light spectra for Ra-223 and F-18 solutions were similar, but those in the vials’ air parts were different. The optical signals in the solution part of both Ra-223 and F-18 are attributed to Cerenkov light, while that in the air part of Ra-223 is attributed to the scintillation of N₂ gas in air. No obvious luminescence of water by the alpha particles was observed. We concluded that optical imaging of the Ra-223 solution was possible, and this solution's light intensity was much higher than that of the F-18 solution and thus easier to detect. Therefore, Ra-223 is a promising radionuclide for optical imaging in distribution measurements.