The discovery of X-rays by Wilhelm Röntgen demonstrated the feasibility of using X-rays for medical diagnosis and subsequently led to intensive research of X-ray imaging technology. X-ray imaging results from differences in the X-ray photons energy reaching the X-ray detector, owing to the different absorption coefficients of X-rays by the object. An X-ray imaging system consists of two main components: the X-ray source and the X-ray detector. A comprehensive overview of the development of X-ray imaging and its shortcomings will facilitate the advancement of X-ray imaging and thus provide a fundamental basis for development of high-performance X-ray detectors.Recently, a new article, published in Research, reviews the developments in X-ray imaging over the past 100 years, and provides an outlook on the future development and challenges of X-ray imaging technology.
This paper begins with the introduction of film-screen radiography. Since the beginning of the 20th century, photographic films were used for a long time as a recorder of X-ray images and scintillator screens as a converter of X-ray energy can effectively reduce the dose of X-ray radiation. After an X-ray exposure, the film is subjected to a series of complex chemical processes (including developing, fixing, washing, and drying) to obtain the final image, which is time and labor-consuming. In 1983, Fuji developed computed radiography (CR), initiating the age of digital 2-dimensional X-ray imaging. However, CR is not able to achieve dynamic imaging due to its separated readout mechanism. The advent of digital radiography (DR) in the early 1990s allowed for dynamic imaging and has become the most widely used X-ray imaging at present. Flat-panel X-ray detectors can be divided into direct and indirect flat-panel detectors, of which, indirect flat-panel detectors are mainly composed of a scintillator layer, an amorphous silicon photoelectric conversion layer, and a thin-film transistor array.
Despite many significant advances in X-ray imaging, the development of low-dose, high-resolution, large-area, and flexible X-ray detectors are still a challenge for researchers. Scintillators, converting high-energy radiation (X-rays, γ-rays, β-particles, etc.) into UV-visible light, play an important role in X-ray detectors. The research on highly efficient scintillators is one of the keys to the manufacture of high-performance X-ray detectors. Currently, CsI:Tl and Gd2O2S:Tb are the most widely used scintillators for X-ray detectors, but they suffer from harsh preparation conditions, long afterglow time, low light yield, and the uncontrollable size caused by high-temperature preparation, making it difficult for the preparation of large-area flexible X-ray detectors.
In recent years, scintillators prepared by low-temperature solution methods hopefully provide new avenues for the next generation of low-cost, highly sensitive, and flexible X-ray detectors. For example, perovskites, which have been emerging in photovoltaics and light-emitting displays, offer advantages such as high quantum yields, narrow emission, and short afterglow. Heavy element Pb contained perovskites facilitate their X-ray absorption and conversion. The feasibility of perovskite nanocrystals as scintillator layers for flat-panel detectors has been demonstrated, and the flexibility in structure and composition has led to a wide range of properties, with research focusing on zero-, two- and three-dimensional non-lead perovskites for X-ray imaging. Recently, a series of rare-earth ions-doped fluorides featuring persistent luminescence were used to fabricate flexible X-ray imaging, a breakthrough in X-ray imaging. Therefore, novel high-efficiency scintillators are expected to break the bottleneck of X-ray imaging and develop the next generation of high-performance X-ray detectors.
From Xinhua