Validation of GATE simulations of the 176Lu intrinsic activity in LSO detectors

The effects of the intrinsic activity of lutetium-based scintillators such as lutetium oxyorthosilicate (LSO) used in positron emission tomography (PET) imaging have been well documented and is generally not a concern in routine scanning. However, this intrinsic activity can become problematic when using a wide energy window or in low count rate scenarios such as cell trafficking studies in small animal imaging. To date, no systematic validation of Monte Carlo simulations of the intrinsic ¹⁷⁶Lu activity has been performed, making it difficult to incorporate them into the design and simulation of proposed scanners. This study seeks to validate Geant4 Application for Tomographic Emission (GATE) simulations of the ¹⁷⁶Lu intrinsic activity in LSO based detectors against data gathered from a pair of LSO-based Siemens Inveon detectors. Measurements from two opposing detector modules were acquired using NIM electronics and a PC based data acquisition (DAQ) card. The detectors were characterized by determining the count rate due to intrinsic coincidence events vs. detector separation while stepping the lower level discriminator (LLD). Monte Carlo simulations were performed using GATE to reproduce the geometry of the bench-top measurements made with the two detectors, modeling the intrinsic activity of the ¹⁷⁶Lu as an ion source located within the scintillator crystals. Initial measurements show good agreement between the simulated and measured results. Intrinsic coincidence count rates are in good agreement at all distances, diverging when the LLD is stepped to a level near an intrinsic photopeak. The bench-top setup results require minor refinement to improve the accuracy of measurements at low energy levels which will be done before completing validation of the simulation results.