1. A computerized tomographic system for measuring characteristics of and imaging, body tissue comprising: scanning means which generate projections of said tissue by directing a known spectrum of radiation through said body and measuring the spatial intensity distribution of radiation traversing said tissue along a multiplicity of paths; image generating means which calculate the attenuation of each pixel element of a raw image array, which is representative of a tranverse section of said tissue, from said projections; means which analyze values in said raw image array and assign to each pixel element, on the basis of those values, a specific proportion of the attenuation coefficient to each of two or more constituents of said tissue; means which combine said element proportions to generate a multiplicity of constituent projections for each of said constituents of said tissue; means which combine said constituent projections, in accordance with a predetermined function approximating the non-linear interaction of said constituents with said spectral intensity distribution, to generate a multiplicity of error projections of said tissue; means which combine said error projections to generate an error image array which is representative of the polychromatic aberration in said raw image array; and means for subtracting said error image array from said raw image array to generate a compensated image array.