1. A method for estimating linear attenuation coefficients at a particular photon energy for a three-dimensional region of a body comprising the steps of: scanning the three-dimensional region with an x-ray beam at a first kVp setting to generate a first CT image data set of values that indicate radiodensity at points in the three-dimensional region; scanning the three-dimensional region with an x-ray beam at a different kVp setting to generate a second CT image data set of values that indicate radiodensity at points in the three-dimensional region; calculating the differences in corresponding values in the first and second CT image data sets; estimating from the calculated differences portions of the three-dimensional region that are relatively low molecular weight material; identifying from the calculated differences portions of the three-dimensional region that are relatively high molecular weight material; and estimating linear attenuation coefficients for the three-dimensional region using one of the first and second CT image data sets and a first conversion factor for the relatively low molecular weight material and a second conversion factor for the relatively high molecular weight material.