Monte Carlo simulation of the ISOLPHARM gamma camera for Ag-111 imaging

TL;DR

This study presents a Geant4-based Monte Carlo model of a dedicated gamma camera optimized for $^{111}$Ag imaging in targeted radionuclide therapy. The camera uses a tungsten square-hole collimator, a GAGG(Ce) scintillator array, and a SiPM readout, achieving a spatial resolution around $4.0\pm0.2$ mm and a sensitivity near $18.6\ \mathrm{cps/MBq}$. Analytical estimates and MC simulations show the device can resolve lesions at a lesion-to-background ratio of $4:1$, demonstrated with MOBY phantom scans of tumor-like inclusions. The work supports the feasibility of low-cost, preclinical imaging of $^{111}$Ag distributions and provides a digital twin framework for optimization prior to hardware construction.

Abstract

Targeted Radionuclide Therapy (TRT) is a well-established technique for cancer treatment. In this approach, radionuclides are bound to specific drugs that selectively transport them to the tumor site. Within the ISOLPHARM project, a radiopharmaceutical for TRT based on the innovative radionuclide Ag-111 is currently under development. Ag-111 has a half-life of 7.45 days and decays by emitting both electrons and gamma-rays. The emission of gamma-rays, predominantly at an energy of 342 keV, enables the visualization of Ag-111 using a gamma camera. In this work, we describe a Monte Carlo simulation developed to optimize the design parameters of such an imaging device. The simulation is based on the Geant4 toolkit, which accurately models the interactions between particles and matter. The estimated spatial resolution and sensitivity of the system are approximately 4 mm and 19 cps/MBq, respectively. The simulated device is able to resolve lesions with a lesion-to-background activity ratio of 4:1 under in-vivo-like conditions. These results indicate that the proposed gamma camera can provide cost-effective imaging capabilities for preclinical radiopharmaceutical studies.

Figures (5)

• Figure 1: Gamma camera scheme: a gamma-ray emitted by the radioisotope passes through the collimator and is converted into optical photons in the scintillator array, these photons cross the coupling volume and are collected in the SiPM.
• Figure 2: Graphical representation of the simulated gamma camera. Blue tracks correspond to gamma-rays, while gray tracks indicate scintillation photons.
• Figure 3: Simulation of a collimated 342 keV gamma-ray beam with an 8 mm diameter impinging directly on the scintillator (without collimator). Left: distribution of the centroid position reconstructed from the energy deposition in the scintillator (information not accessible in a real experiment). Right: reconstructed position obtained from SiPM signals with the center-of-gravity algorithm.
• Figure 4: 1D spatial distribution of 342 keV gamma-rays generated in a line along the x-axis (with $x \in [-5,5]\,\mathrm{mm}, y = 0\,\mathrm{mm}, z = 30\,\mathrm{mm}$) that passed through the central hole of the collimator.
• Figure 5: Simulated images with two tumors 10 mm apart inside a mouse model. The assumed lesion-to-background activity concentration is 4:1.