Evaluation of a Compton Camera System for Whole-body Imaging of Ac-225: A Simulation Study

In this study, we introduce a Compton camera system design for whole-body imaging of Actinium-225 (225Ac), one of the trending radionuclides for targeted alpha therapy (TAT). The system enables multi-energy gamma photon detection with higher efficiency compared to mechanically collimated SPECT. This design consists of two detectors made of Cadmium zinc telluride (CZT), providing a field of view (FOV) adequate for whole-body imaging, while achieving high sensitivity and clinically usable imaging resolution within a reasonable scanning time. This work focuses on the system design and evaluation using the Monte Carlo simulation toolkit GATE. The imaging performance is evaluated at two energy peaks (218 keV, 440 keV) by applying the energy windows of 211-225 keV and 430-450 keV, representing the major detectable gamma energies generated from 225Ac. We explore the possibility of using the Compton camera system for treatment response monitoring in TAT. The full decay chain of 225Ac is simulated. Results demonstrate an image resolution of 1.0 cm using a NEMA IQ phantom with 5.7 MBq of 225Ac simulated in a cold background. An image resolution of 1.3 cm can be achieved with a hot-to-background ratio of 30:1, and a resolution of 3.7 cm can be achieved with an activity ratio of 12:1. The best achievable sensitivity at a 10 cm distance from the detector is 0.5% at 218 keV and 0.45% at 440 keV as calculated using a pure gamma source in the simulation. The proposed system may serve as an alternative imaging tool for TAT scanning in clinical settings.