Optimization of Large-Angle Pinhole Collimator for Environmental Monitoring System

The purpose of this study was to optimize a large-angle pinhole collimator using Monte Carlo simulation for nuclear survey imaging. Simulations using GATE (Geant4 Application for Tomographic Emission) were performed to model the pinhole gamma camera system. A gamma camera consists of a cone-shaped pinhole collimator with a tungsten aperture and a CsI(Tl) scintillation crystal 6.0 mm thick and 50.0 mm x 50.0 mm in area. The focal length and the acceptance angle of the pinhole collimator were set to 14.5 mm and 120 degrees, respectively. The intrinsic spatial resolution and sensitivity were simulated by changing the pinhole diameter and channel height. The point source of Tc-99m was located 30.0 mm above the center of the pinhole, and the projection data was estimated for pinhole diameter values from 0.5 mm to 4.0 mm while the channel heights were fixed between knife-edge and 3.0 mm. The optimal ranges of channel height and pinhole diameter were determined through evaluation of the intrinsic resolution and sensitivity tradeoff curves. The simulation results allowed us to determine the optimal values of pinhole diameter and channel height to be 1.5 mm and 0.5 mm, respectively, to get intrinsic resolution below 2.0 mm FWHM with a reasonable sensitivity for the system configured in this study. The resolution and sensitivity were measured experimentally, and the simulated and measured data were in good agreement. The results demonstrated that the pinhole collimator designed in this study could be utilized to create a large-angle radiation monitoring system.