Abstract: A method and apparatus is provided to calculate scatter using a method to determine primary X-ray flux, first-scatter flux, and multiple-scatter flux using an integral formulation of a radiative transfer equation and using spherical-harmonic expansion. The integral for the primary X-ray flux does not include a spherical-harmonic expansion. The integral for the first-scatter flux includes an angle-dependent scatter cross-section. The integral for the multiple-scatter flux is performed iteratively, includes spherical harmonics, and includes a scatter cross-section expanded using Legendre polynomials. The integrals of attenuation along propagation rays can be accelerated using material decomposition of the attenuation coefficients. An anti-scatter-grid term can be included in the integrals to account for the effects of an anti-scatter grid on the fluxes prior to detection of the X-rays.