kV beam model for flat panel imaging system with bow tie filter

Abstract

Purpose: Kilovoltage flat-panel imaging systems are used for cone-beam Computed Tomography (CBCT) and digital Tomosynthesis (DTS). Hereby, the presence of scatter and relatively large dose from imaging are challenging factors. In this project a phenomenological beam model was developed to characterize imager response to imaging beams with a bow tie filter (Varian OBI system). Materials and Method: The kilovoltage beam model is based on dose ratio formalism and thus is using standard concepts of megavoltage dose calculation such as scatter factors, tissue maximum ratio and off-axis ratio. Primary and scatter (head and phantom scatter) are modeled with three Gaussian kernels. Parameters were derived with slabs of solid water and various jaw settings. Results: The beam model was used to evaluate contributions from primary, secondary and tertiary photons for different geometrical objects such as cylinders and steps of solid water. Predictions of radiographs using the model for known objects are consistent with the measurements. Conclusion: Secondary and tertiary contributions were interpreted as scatter and can be subtracted from CBCT projections. Therefore our model can provide a basis for improvement of image quality (less artifacts due to scatter, better contrast and resolution) in CBCT reconstruction.