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 technique further includes performing a set of Monte Carlo simulations. The method further includes performing a principal component analysis on the set of Monte Carlo simulations to generate a principal component analysis model of the detector data. The method also includes estimating the density of the material at one or more locations within the annular space based upon the principal component analysis model and the detector data.