AbstractID: 7808 Title: Analysis and correction of scattering for mega-voltage (MV) cone beam computed
tomography (CBCT) using Monte Carlo simulation
Scattered photons are a significant contribution to radiation detected by an EPID in mega-voltage (MV) imaging. Such scatter
contribution is particularly important in MV cone-beam CT (CBCT) now being considered for applications in patient localization. We
use Monte Carlo simulation to analyze and correct for scattering encountered in MV-CBCT. Diagnostic CT images were imported to
construct a phantom of three-dimensional electron density values. The fluence incident on the phantom was obtained by convolving
the beam aperture with the known geometrical and spectral distribution of the x-ray (primary and scatter) sources. The primary
photon fluence reaching the EPID was calculated analytically while photons generated by phantom scatter were simulated using EGS4
and transported and tallied by an in-house user code. To simulate MV-CBCT, we rotated the source around the phantom and
calculated projection images at gantry angles separated by 2° obtaining 100 images for volume reconstruction. The spatial scattering
distribution fit well to a second order polynomial (R2≈0.98). For the phantom and source-to-detector distance used here the ratio of
scatter to primary contributions is 1/2. Although scatter does not significantly affect image quality for individual projection images,
the MV-CBCT images reconstructed without scattering correction are inferior (lower contrast, lower CT number, and higher noise) to
those with scattering correction. The scattering correction also reduces the so-called “cupping” effect, an artificial reduction in CT
number with radial distance. Thus scattering should not be ignored for MV-CBCT, and a simple second-order polynomial scattering
correction can significantly improve reconstruction accuracy.