Dual-energy X-ray imaging: benefits and limits

Both in radiographic and tomographic mode, conventional X-ray imaging provider information about the examined object which is not sufficient to characterise it precisely. The dual-energy X-ray technique, which works by combining two radiographs acquired at two distinct energies, permits to obtain both density and atomic number, thus information about material composition. Available systems usually perform energetic separation at the source level, but separation at the detector level is also possible,for linear detectors, especially those devoted to translating objects control. Dual-energy equations can be easily written and solved for monochromatic energy spectra and perfect detectors, but become complex when considering realistic spectra, detector sensitivity, and system non-linearity. The decomposition onto a material basis, using experimental dual-material calibration, allows an approximated system to be solved, while eliminating the effects of disturbances such as beam hardening. More generally, we analyse the various problems to be solved before benefiting from the dual-energy approach, and propose available solutions. We evaluate the influence of noise on the accuracy of results, which strongly influences the capability to distinguish materials. We review the aspects to be optimised when considering a specific industrial problem. Numerical simulation is an efficient tool for optimal system design. Various industrial applications are considered.