A novel preclinical system for cascade gamma photon coincidence imaging with high resolution and sensitivity

Objective. Cascade gamma-emitting radionuclides such as¹⁷⁷Lu offer unique opportunities for theranostic applications, yet existing imaging systems face inherent trade-offs between spatial resolution and detection sensitivity. To address this limitation, we present a novel time-space coincidence imaging system specifically designed for cascade gamma photons, aiming to achieve both high spatial resolution and high sensitivity in a single platform.Approach. The proposed system employs a full-ring LaBr₃detector array combined with hybrid pinhole-slit collimators, which synergistically integrate the high spatial resolution of pinhole collimation with the high detection efficiency of slit collimation. The imaging performance was evaluated through Monte Carlo simulations using both a point source and a Derenzo phantom, with¹⁷⁷Lu as the radionuclide source, which emits cascade gamma-ray pairs at 113 keV and 208 keV. Image reconstruction was performed using direct back-projection (DBP) and maximum likelihood expectation maximization (MLEM), alongside two newly developed algorithms: refined DBP (R-DBP) and multi-information joint reconstruction (MIJR).Main results. The system achieved a central coincidence sensitivity of 2.97 × 10^-5and an average spatial resolution of 2.32 mm. In Derenzo phantom imaging, hot rods with diameters as small as 1.2 mm were clearly resolved, and the contrast-to-noise ratio (CNR) for the largest rods reached 9.29. The use of the R-DBP algorithm effectively reduced image artifacts. Compared with the DBP algorithm, the CNR of the reconstructed images was improved by an average of 4.37%. Under low-count conditions, the MIJR algorithm achieved an average CNR improvement of 3.80% over the MLEM algorithm. These quantitative results demonstrate that the hybrid collimator design effectively balances resolution and sensitivity, outperforming conventional cascade gamma imaging approaches.Significance. These results demonstrate that the proposed hybrid collimator design provides a favorable balance between spatial resolution and coincidence sensitivity, achieving better spatial resolution than previously reported systems while maintaining higher sensitivity than conventional collimated approaches. By combining high-resolution imaging, practical sensitivity, and inherent theranostic capability with¹⁷⁷Lu, this technology provides a promising platform for integrated diagnosis and therapy in preclinical and potentially clinical applications.