Duo-Spectral Imaging with Multilayered Energy-Integrating Detectors

Generally, X-ray flat-panel detectors (FPDs) are based on amorphous silicon or complementary metal–oxide–semiconductor technology. In principle, they operate in the energy-integration mode, with their response proportional to the average energy of the detected spectral beam. Thus, the energy integrator weighs X-ray photons unfairly and places more weight on the higher-energy photons. Recent photon-counting detectors (PCDs) can operate with the same weight, regardless of the photon energy, and with more sophisticated weighting schemes. In particular, the development of FPDs has accelerated the adoption of digital X-ray techniques in medicine and other industries. Moreover, PCDs are emerging as the next-generation technology that renders colored images—a feature that cannot be achieved using black-and-white energy-integrating detectors. The development of PCDs should improve the availability of large-area imaging at a reasonable cost to replace the current role of FPDs in radiology. Similar to PCD imaging but with limited spectral decomposition, a linear combination of FPD images recorded at two different energies can present energy-resolved images. The resulting image quality is inadequate because of noise amplification; however, lesion conspicuity can be enhanced by suppressing background clutter. Moreover, instead of performing two measurements at two separate exposures, dual-energy imaging (DEI) can be performed at a single exposure by stacking two FPDs, similar to a sandwich. Therefore, single-shot DEI is intrinsically immune to motion artifacts. Furthermore, a sandwich design with FPDs featuring different spatial-resolution characteristics enhances the edges of the images. This chapter presents a brief review of sandwich-detector DEI with the operating principle described using a simple linear model. Demonstrations of DE images obtained using laboratory-scale compact-form sandwich detectors are presented for possible applications in radiography, angiography, and tomography. Hence, applying FPDs to sandwich-detector DEI at reasonable costs, facilitated by stiff competition in the FPD business, may serve as a niche before the widespread use of PCDs in the future.