Experimental verification of dual-isotope imaging of 225Ac/213Bi using alpha imaging detector

Understanding the spatial distribution of alpha-emitting isotopes is important for advancing Targeted Alpha Therapy (TAT) research, particularly in identifying the biokinetics of alpha-labeled radiopharmaceuticals in pre-clinical animal experiments. However, challenges persist in alpha imaging, particularly in investigating the behavior of recoiled nuclides and long-lived daughter nuclides such as ²¹³Bi in the decay chain of ²²⁵Ac. In this study, we demonstrate dual-isotope imaging of ²²⁵Ac/²¹³Bi, through experimental validation. Building upon our previous Monte Carlo simulation study, the dual-isotope imaging is a method to spatially resolve ²²⁵Ac and ²¹³Bi using energy spectrum deconvolution technique. An alpha imaging detector, developed for TAT applications, comprises a GAGG(Ce) crystal, a silicon photomultiplier (SiPM), and a dedicated digital data acquisition system. The developed system features good energy resolution (3.9 % at 5 MeV alpha), allowing distinction of alpha particles according to their energy signatures while imaging, thereby facilitating the identification of the source isotope. The capability of the alpha imaging detector in spatially resolving ²²⁵Ac and ²¹³Bi was demonstrated on a source made of a thin-layer chromatography (TLC) where ²²⁵Ac-DOTATATE was separated. From the measurement of the TLC strip using the alpha imaging detector, the ²²⁵Ac/²¹³Bi ratio was estimated in distinct regions using two methods and compared: an analytic method using decay characteristics and the energy spectrum deconvolution method. The results demonstrated that using the dual-isotope imaging, amounts of ²²⁵Ac and ²¹³Bi in each region can be estimated as those estimated by the analytic method, even without repeated measurements. By distinguishing the origin of alpha particles from the image, we expect the dual-isotope imaging to provide a tool for better understanding the behavior of recoiled daughter nuclides from radiopharmaceuticals in animal bodies.