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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.
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