Impact of CT Simulation Parameters on the Realism of Virtual Imaging Trials

Virtual imaging trials (VITs) provide the opportunity to conduct medical imaging experiments otherwise not feasible through patient images. The reliability of these virtual trials is directly dependent upon their ability to replicate clinical imaging experiments. The combined effect of various key simulation parameters on the closeness of virtual images to experimental images has not yet been explicitly quantified, which this sensitivity study aimed to address. To do so, a physical phantom, Mercury 3.0 (Sun Nuclear), was scanned using a clinical scanner (Siemens Force). Meanwhile, utilizing a validated CT simulator (DukeSim), a computational version of the Mercury 3.0 phantom was virtually imaged, emulating the same scanner model and imaging acquisition settings. The simulations were performed with varied parameters for the x-ray source, phantom model, and detector characteristics, evaluating their impact on the realism of the final reconstructed virtual images. Simulations were explicitly conducted and evaluated various source and detector subsampling (1 – 5 per side), phantom voxel resolution (0.1 mm – 0.5mm), anode heel severity (0% - 40% over anode-cathode axis), aluminum filtration (0.9cm - 1.1cm), and pixel-to-pixel detector crosstalk (0 – 10.5%, 0 – 15% per dimension). The real and simulated projections were then reconstructed, employing a vendor-specific reconstruction software (Siemens ReconCT), with identical reconstruction settings. The real and simulated images were then compared in terms of modulation transfer function (MTF), noise magnitude, noise power spectrum (NPS), and CT number accuracy. When the optimal simulation parameters were selected, the simulated images closely replicated real images (0.80% relative error in f50air metric). The error in the f50 measurements were highly sensitive to the variation of source and detector subsampling and phantom voxel size. The relative error in the noise magnitude measurements were not highly sensitive to the variation of source and detector subsampling or phantom voxel size but were sensitive to the modeling of the anode heel effect severity. The error in the nNPS measurements were not highly sensitive to the variation of source and detector subsampling, phantom voxel size, degree of anode heel severity, aluminum filtration, or detector cross talk. Finally, the error in the CT number accuracy measurements were not highly sensitive to the variation of source and detector subsampling, phantom voxel size, aluminum filtration, or degree of detector cross talk, but were sensitive to the modeling of anode heel severity. Through this study, the effects of various key simulation parameters on the realism of scanner-specific simulations were assessed. Certain simulation parameters, such as source and detector subsampling, and degree of anode heel severity, exert greater influence on simulation realism than others, thus they should be prioritized when exploring novel modeling avenues.