In Vivo Study of Total-Body Kinetics of Ac-225 and Its Daughters in Mouse Using a CZT-Based Small Animal SPECT-CT System
Abstract
Purpose
We present a small-animal SPECT-CT system based on CZT imaging spectrometers and a rapidly switchable dual-FOV apertures. The system supports both total-body imaging and focused, high-resolution imaging. These capabilities address a key need in preclinical radiopharmaceutical therapy development for sensitive, high-resolution assessment of in vivo tracer kinetics over multiple time points with CT-based anatomical co-registration.
Methods
A mouse received an intravenous injection of Ac-225 labeled antibody (4 µCi), and SPECT-CT scans were acquired at 4 h, 24 h, 48 h, and 72 h post-injection to characterize time-dependent Ac-225 and daughters distribution. At each time point, SPECT data were acquired for 60 minutes with a projection bin size of 1mm × 1mm and reconstructed using OSEM with 200 iterations and 8 subsets. CT was acquired for anatomical reference, enabling organ-level localization of the reconstructed SPECT activity and consistent cross-time-point alignment.
Results
SPECT-CT provided clear visualization and quantification of Ac-225 and daughters' distribution across the whole body. Serial imaging demonstrated systematic, time-dependent changes in activity concentration from 4 h to 72 h post-injection. Higher early daughter activity concentrations were observed in the bladder and kidneys, followed by reduction at later time points, whereas liver activity concentrations remained relatively stable with limited temporal variation. CT-based co-registration enabled organ-specific localization of SPECT activity, supporting qualitative evaluation of tracer clearance kinetics over time.
Conclusion
A CZT-based small-animal SPECT-CT system was evaluated. Serial SPECT-CT imaging of Ac-225 and daughters revealed a clear washout process across multiple post-injection time points. These results indicate that the system is well suited for quantitative and kinetic studies of alpha-emitter radiopharmaceuticals in small animal.