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 wherein the differences are calculated between points in the first and second CT image data sets that correspond to the same physical location in the three-dimensional region; estimating from the calculated differences portions of the three-dimensional region that are relatively low molecular weight material; estimating 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, wherein the first conversion factor is different from the second conversion factor.