Quantitative imaging of chemical composition in patients may be a very useful diagnostic technique for a variety of condidtions, such as iron overload in the liver - a serious and potentially life-threatening chronic disease. We have developed a method to noninvasively measure the quantity of a specific element in vivo. This method is based upon the fact that very different atomic numbers and density can be discriminated by x-ray beams of different energies. The main advantage, and inventive step, of this method (relative to techniques used in the 1980s) is that the processing is performed on the reconstructed CT images, making the technique easy to implement and fast to execute. This technique also can be used in any general quantitative imaging application for which only two materials are considered.