Compton and Proximity Imaging of Ac In Vivo With a CZT Gamma Camera: A Proof of Principle With Simulations

Abstract

In vivo imaging of 225 Ac is a major challenge in the development of targeted alpha therapy radiopharmaceuticals due to the extremely low injected doses. In this article, we present the design of a multimodality gamma camera that integrates both proximity and Compton imaging in order to achieve the demanding sensitivities required to image 225 Ac with good image quality. We consider a dual-head camera, each of the heads consisting of two planar cadmium zinc telluride detectors acting as scatterer and absorber for Compton imaging, and with the scatterer practically in contact with the subject to allow for proximity imaging. We optimize the detector's design and characterize the detector's performance using Monte Carlo simulations. We show that Compton imaging can resolve features of up to 1.5 mm for hot-rod phantoms with an activity of 1 µCi, and can reconstruct 3-D images of a mouse injected with 0.5 µCi after a 15-min exposure and with a single bed position, for both 221 Fr and 213 Bi. Proximity imaging is able to resolve two 1 mm-radius sources of less than 0.1 µCi separated by 1 cm and at 1 mm from the detector, as well as it can provide planar images of 221 Fr and 213 Bi biodistributions of the mouse phantom in 5 min.

A Generalization of the Maximum Likelihood Expectation Maximization (MLEM) Method: Masked-MLEM

“…6), indicating that some collimation should be required to provide more accurate position information for reconstruction. Though reduced reconstruction quality, we experimentally demonstrate the feasibility of performing proximity reconstruction with a Compton camera system, as discussed in [30]. How to improve the image reconstruction quality in collimatorless imaging will be investigated in our future studies.…”

A generalization of the maximum likelihood expectation maximization (MLEM) method: Masked-MLEM

“…Furthermore, due to the dead stripes in the detector (see figure 7), certain valuable projection data are missing, degrading the image quality. Though reduced reconstruction quality, we experimentally demonstrate the feasibility of performing proximity reconstruction with a Compton camera system, as discussed in Caravaca et al (2022).…”

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

“…Other works have already proposed a Compton camera for the preclinical evaluation of the daughter 221 Bi; 24,25 however, none have demonstrated the feasibility of this modality via experimentation. The most recent study by Caravaca et al 25 simulates the response Previous works have proposed and employed coded apertures for small-animal imaging, 26 but as far as we know, this work is the first to recommend a coded aperture for imaging 221 Fr via its 218-keV emission in small animals. Other analogous collimator-based systems have been employed to image 221 Fr.…”

“…Other works have already proposed a Compton camera for the preclinical evaluation of the daughter 221 Bi; 24,25 however, none have demonstrated the feasibility of this modality via experimentation. The most recent study by Caravaca et al 25 . simulates the response of a Compton imager based on CZT.…”