1. An x-ray imaging system, comprising: an x-ray source system for generating a high energy x-ray beam and a low energy x-ray beam; a sample holder for rotating a sample in the high energy x-ray beam and the low energy x-ray beam from the x-ray source system to perform a low-energy scan and a high-energy scan of the sample; a detector system for detecting the high energy x-ray beam and the low energy x-ray beam after transmission through the sample to generate projection data; a computer system generating a low-energy reconstructed tomographic volume data set from the low-energy scan and generating a high-energy reconstructed tomographic volume data set from the high-energy scan from the projection data, rendering simultaneous slice views from the low-energy and high-energy reconstructed tomographic volume data sets and presenting a mathematically combined synthetic slice, presenting an interactive graphic for displaying a two-dimensional slice selection from the low-energy and high-energy reconstructed tomographic volume data sets, rendering a low-energy slice view from the low-energy reconstructed tomographic volume data set and a high-energy slice view from the high-energy reconstructed tomographic volume data set for the same region of the sample, in response to user selection of a slice view within the two-dimensional slice selection graphic, creating a two-dimensional histogram of high-energy x-ray pixel intensity values versus low-energy x-ray pixel intensity values based on the selected slice in which intensity of points on the histogram is associated with a count of voxels with the low-energy and high-energy pixel intensity values respectively, and displaying the histogram.