Monte Carlo modelling of cone beam collimation with scattering in objects [SPECT application]

A novel Monte Carlo model of low-energy photon detection is proposed for cone beam collimation. The model is based on the existing analytic formulas of collimator resolution and sensitivity for primary photons as well as forced detection techniques for parallel beam collimation. The collimator resolution for scattered photons is simulated by forcing photons scattered into elliptical acceptance cones oblique to collimator surface. The sensitivity for the scattered photons, used as a weight for variance reduction, is derived based on the sensitivity for primary photons, the Klein-Nishina distribution, and the elliptical acceptance cone subtended by the circular contour of a point response with a radius of FWHM resolution. A circular cone approximation of the elliptical acceptance cones is proposed to increase computational efficiency for the forced detection procedure. The Monte Carlo model with the circular cone approximation is validated by comparing simulated projections with experimental data. The comparison demonstrates the accuracy of the Monte Carlo model of cone beam collimation