SU‐E‐J‐21: Verification of a Monte Carlo Model of CBCT Flat Panel Detector Using BEAMnrc/EGSnrc Code

Purpose: Cone beam computed tomography (CBCT) provides wide scan coverage per rotation; however, its image quality is compromised due to large amounts of scatter. In this study, we performed detailed Monte Carlo (MC) simulations of a CBCT flat panel detector to characterize scatter for the purposes of scatter correction. Methods: An amorphous silicon (aSi) flat panel detector (Varian Medical Systems) of an On‐Board Imager (Varian Medical Systems) was modeled using BEAMnrc/EGSnrc code system based on detailed geometric information provided by the manufacturer. Layers from the proximal Al cover to the light reflector encompassing the 10:1 anti‐scatter grid were simulated using the block component module (BLOCK_CM) in BEAMnrc. Layers from the cesium iodide (CsI) detector to the proximal Pb electronics protection cover were modeled in DOSXYZnrc to create a voxelized representation of the detector layer. Various scatter properties were elicited from phase space files within the grid and detector layers. A two‐dimensional (2‐D) cone‐beam image “in‐air” (without the phantom) was acquired (125 kVp, 80 mAs). 2‐D and 1‐D pixel intensities were compared to the simulated projection to verify the accuracy of MC simulation of the entire detector system. Results: 2‐D pixel intensities of the computed image agreed well to the measured image as the difference map showed values within +−10%. However, given the large number of histories required for detector simulation, the MC uncertainty was quite high, up to 10% in some regions. The central axis profile also showed good agreement between simulation and measurement within 3% on average, except in the regions where the statistical uncertainty was larger (1sigma=7.5%). Conclusions: A CBCT imager has been modeled in detail using the BEAMrnc/EGSnrc code system. Additional verification of the MC‐based modeling is warranted to for the purposes of scatter characterization.