Performance evaluation of a Compton SPECT imager for determining the position and distribution of ²²⁵Ac in targeted alpha therapy: A Monte Carlo simulation based phantom study

In this study, the performance of a Compton Single Photon Emission Computed Tomography (SPECT) imager when in vivo monitoring the position and distribution of ²²⁵Ac radionuclide in targeted alpha therapy (TAT) was evaluated. When ²²⁵Ac radionuclide, which emits various γ-rays (218 and 440 keV), is used in TAT, both the photoelectric and Compton scattering events can be used for image reconstruction. Moreover, all information pertaining to the various γ-rays of the ²²⁵Ac radionuclide can be individually or simultaneously utilized in the reconstructed image. Three types of simulation phantoms and a quantitative evaluation method were used to compare the performance of the Compton SPECT imager to that of conventional SPECT imaging, which uses only photoelectric events, and the results demonstrated that the Compton SPECT imager exhibited a higher performance as the effective count for the image reconstruction was higher. To verify the accuracy of the position and distribution of the ²²⁵Ac radionuclide that had been inserted into the phantom, reconstructed images of the various γ-rays were combined with cross-sectional images of the human phantom and all combined images were found to match the predetermined simulation conditions. In conclusion, the simulation results demonstrated the feasibility of the in vivo monitoring of the position and distribution of ²²⁵Ac radionuclide using the γ-rays in TAT.