[en] An inherent problem in the analysis of X-ray computerised tomography is that the X-ray beam incident upon the patient is not monoenergetic. In the past, the use of waterbags has allowed image reconstruction as though the measurements were made with an effectively monochromatic X-ray beam. A novel solution is described which takes advantage of the known functional dependence of X-ray attenuation cross sections on the X-ray energy and atomic number. The incident X-ray spectrum is changed by either altering the tube voltage or introducing a suitable filter; equivalently, the efficiency of the detector over different energy ranges may be changed. The two measurements can then be processed to yield an effectively monoenergetic scan. A complex mathematical derivation is given. (U.K.)