Purpose: A novel dual-layer on-board imager offering dual energy processing capabilities is being designed. A computational framework is required to optimize the design parameters and evaluate detector performance for specific clinical applications. In this study, we report on the development of a Monte Carlo (MC) model of the imager and model validation.
Methods: The stack-up of the dual-layer imager (DLI) was implemented in GEANT4 Application for Tomographic Emission (GATE). The physics model includes electromagnetic interactions and transport of both x-rays and electrons. The DLI model has an active area of 43×43 cm², with top and bottom Cesium Iodide (CsI) scintillators of 700 μm and 800 µm thickness, respectively. The Modulation Transfer Function (MTF) of the detector was calculated for a 140 kVp x-ray spectrum using a 0.5 mm thick tantalum edge at 2.6ᴼ. For validation of the MC approach, measured and simulated data for a single layer PaxScan 4030CB flat-panel detector (600µm) was compared, using an 80 kVp x-ray spectrum. Optical processes in the scintillator and columnar crystal structure were included in the validation simulations. In the absence of manufacturer’s information on the size of the crystal, a columnar diameter of 150µm was used with the surrounding volume filled with nitrogen gas.
Results: The MTF of the bottom layer of the dual-layer model shows values decreasing faster with spatial frequency, compared to the top layer, due to the thicker bottom scintillator thickness and scatter from the top layer. In the validation study, inclusion of optical photon generation and transport in the simulation results in a good agreement between measurement and simulations especially at higher spatial frequencies.
Conclusion: A first computational model of a prototype dual-layer flat panel detector for kV-CBCT was developed using the GATE toolkit. The initial results indicate a small degradation in resolution for the thicker bottom layer.