Correction of scatter in megavoltage cone-beam CT

The role of scatter in a cone-beam computed tomography system using the the rapeutic beam of a medical linear accelerator and a commercial electronic p ortal imaging device (EPID) is investigated. A scatter correction method is presented which is based on a superposition of Monte Carlo generated scatt er kernels. The: kernels are adapted to both the spectral response of the E PID and the dimensions of the phantom being scanned. The method is part of a calibration procedure which converts the measured transmission data acqui red for each projection angle into water-equivalent thicknesses. Tomographi c reconstruction of the projections then yields an estimate of the election density distribution of the phantom. It is found that scatter produces cup ping artefacts in the reconstructed tomograms. Furthermore, reconstructed e lectron densities deviate greatly (by about 30%) from their expected values . The scatter correction method removes the cupping artefacts and decreases the deviations from 30% down to about 8%.