Dual-energy X-ray diagnostics

The X-ray attenuation coefficient of biological materials can be split into separate contributions due to Compton scattering and photoelectric absorption. The contributions are each the product of a known energy-dependent factor and an unknown material-dependent factor. If two X-ray exposures are made at different energies the material-dependent factors due to Compton scattering and to photoelectric absorption can be determined. This enables X-ray images entirely due to Compton scattering or to photoelectric absorption to be calculated. These images can be weighted by the energy-dependent factors and combined to give monoenergetic X-ray images. The contrasts between tissues can then be varied by varying the notional X-ray energy. A calibration procedure is necessary because the polyenergetic X-radiation becomes harder in the object. This procedure provides calibration coefficients for the material-dependent factors. A Philips CT scanner has been modified to supply alternate high-voltage and low-voltage pulses to the X-ray tube.