Coded aperture and Compton imaging for the development of 225 Ac-based radiopharmaceuticals.

Abstract

Targeted alpha-particle therapy (TAT) has great promise as a cancer treatment. Arguably the most promising TAT radionuclide that has been proposed is 225 Ac. The development of 225 Ac-based radiopharmaceuticals has been hampered due to the lack of effective means to study the daughter redistribution of these agents in small animals at the preclinicalstage. The ability to directly image the daughters, namely 221 Fr and 213 Bi, via their gamma-ray emissions would be a boon for preclinical studies. That said, conventional medical imaging modalities, including single photon emission computed tomography (SPECT) based on nonmultiplexed collimation, cannot be employed due to sensitivitylimitations. As an alternative, we propose the use of both coded aperture and Compton imaging with the former modality suited to the 218-keV gamma-ray emission of 221 Fr and the latter suited to the 440-keV gamma-ray emission of 213 Bi. This work includes coded aperture images of 221 Fr and Compton images of 213 Bi in tumor-bearing mice injected with 225 Ac-basedradiopharmaceuticals. These results are the first demonstration of visualizing and quantifying the 225 Ac daughters in small animals through the application of coded aperture and Comptonimaging.