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Comparing experimental measurements of indirect x-ray detector responses with Monte Carlo predictions: figures of merit and model development

Thalium-activated Cesium Iodide (CsI:Tl) scintillator screens coupled with optical readout arrays are currently the most commonly implemented detection method for digital x-ray imaging. The development of Monte Carlo code to provide detailed simulations of detectors has created the need to quantitatively compare Monte Carlo simulated versus experimentally measured point-response functions (PRFs) for validation. MANTIS is a Monte Carlo code developed for the detailed simulation of columnar CsI scintillator screens. In previous work we have validated the ability of MANTIS to reliably predict experimental PRFs and formulated an analytical model of CsI detector response that generates a PRF comparable to that of MANTIS in less than one millionth of the computation time. At the same time, these results demonstrated the need for optimization of MANTIS input parameters and the method for quantitatively comparing Monte Carlo generated PRFs with experimental data. In this study, we explore the figure-of-merit (FOM) used to compare MANTIS and experimental PRFs and the parameter space of the MANTIS inputs. We used high-resolution PRFs experimentally measured using a 30-μm pinhole arrangement. Multiple sets of simulated PRFs based on partial knowledge of the experimental setup and CsI:Tl screen structures have been produced to compare with the experimental measurements pertaining to a screen with a thickness of 170 μm, an x-ray energy spectrum of 40 kVp, and two x-ray beam incidence angles of 0 and 45 degrees. Input parameters of the Monte Carlo code that were not fixed by the experimental setup or previous validation results were varied to produce the different simulated PRFs. We introduced different FOMs to compare simulated and experimental PRFs and used these FOMs to fine-tune MANTIS models to best represent a particular screen design.