A method implemented using one or more computer processors for estimating the density of a material in an annular space includes receiving detector data representative of scattered photons resulting from interaction of a material in an annular space with radiation from a radiation source and detected by a plurality of radiation detectors. The method further includes performing a set of Monte Carlo simulations and generating polynomial models of the detector data based on the set of Monte Carlo simulations. The method further includes estimating the density of the material at one or more locations within the annular space based upon the polynomial models and the detector data.