Secondary radiations in CBCT: a simulation study

Accurate quantitative reconstruction in kV cone-beam computed tomography (CBCT) is challenged by the presence of secondary radiations (scattering, fluorescence and bremsstrahlung photons) coming from the object and from the flat-panel detector itself. This paper presents a simulation study of the CBCT imaging chain as a first step towards the development of a comprehensive correction algorithm. A layer model of the detector is built in a Monte Carlo environment in order to help localizing and analyzing the secondary radiations. The contribution of these events to the final image is estimated with a forced-detection scheme to speed-up the Monte Carlo simulation without loss of accuracy. We more specifically assess to what extent a 2D description of the flat-panel detector would be sufficient for the forward model (i.e. the image formation process) of an iterative correction algorithm, both in terms of energy and incidence angle of incoming photons. A convolution model to account for detector secondary radiations is presented and validated. Results show that both object and detector secondary radiations have to be considered in CBCT.