Monte Carlo modeling of field angle-dependent spectra for X-ray imaging systems

The photon spectrum for X-ray capture systems is a function of the emission field angle. Spectrum variability is the most pronounced for cone-beam computed tomography (CBCT) systems with wide field angles operating close to the anode angle limit Filtration devices also contribute to the change in the photon spectrum with an emission field angle, especially for variable-thickness filters, e.g., bow-tie filters. The change in the photon spectrum is primarily due to the distance traversed through the anode and filtration materials with emission field angles. Although Monte Carlo X-ray simulations can include the materials and geometries for these source assembly elements, the computational requirements are considered prohibitive. As a consequence, most X-ray Monte Carlo simulation implementations ignore emission field angle spectral effects. Our approach uses a probabilistic rejection scheme to model the emission field angle spectral effects within the context of a Monte Carlo simulation tool. A bounding spectrum is constructed that supersedes all possible spectrums, i.e., for all emission field angles. Photons are generated with the bounding spectrum and rejected or accepted based on the probability of transmission through the cascade of anode and filtration materials relative to a pre-calculated maximum probability of transmission. The resultant photon spectrum properly models the intensity and spectral shape of the emitted photons as a function of the emission field angle. The modeling accuracy improvement over the constant spectrum approximation was calculated for a CBCT system for anode voltages ranging from 50 Kvp to 110 Kvp. The maximum improvement in predicted primary and scatter signals was approximately 5% for a system configuration employing a simple filtration and 25% for a CBCT system employing a bow-tie filter with less than a 10% additional computation cost.