Dual-isotope imaging method for Actinium-225 and Bismuth-213 using alpha imaging detector

Recently, ²²⁵Ac has received considerable attention for its use in targeted alpha therapy because it has a relatively long half-life and yields four more alpha-particles from the daughter nuclides. The performance evaluation should separately assess the distribution of ²²⁵Ac and ²¹³Bi because daughter nuclides, including ²¹³Bi, can cause renal toxicity, which may hinder the widespread use of ²²⁵Ac for targeted alpha therapy. In this study, we describe and validate a spectrum decomposition method for dual-isotope imaging of ²²⁵Ac and ²¹³Bi using an alpha imaging detector. We implemented an experiment to demonstrate the feasibility of using the alpha imaging detector to obtain distribution images using therapeutic amounts of ²²⁵Ac. In addition, we designed and conducted a Monte Carlo simulation under realistic conditions based on the experimental results to evaluate the spectrum decomposition method for dual-isotope imaging. The alpha imaging detector exhibited a detection efficiency of 18.5% and an energy resolution of 13.4% at 5.5 MeV. In the simulation, the distributions of ²²⁵Ac and ²¹³Bi were obtained in each region with a relative error of 5%. The results of this study confirmed the feasibility of the dual-isotope imaging method for discriminating alpha-emitters using small amounts of ²²⁵Ac.