Solid-State X-Ray Imagers
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Solid-State X-Ray Imagers
Yoshihiro Izumi and Yasukuni Yamane Abstract New solid-state x-ray imagers known as digital flat-panel x-ray detectors are about to be launched in the field of medical diagnostics. The combination of active-matrix technologies developed for active-matrix liquid-crystal displays and x-ray detection materials constitutes the basis of the detectors. Recently, two kinds of direct-conversion detectors have been developed in order to improve their performance. One is a detector using a thick x-ray photoconductor (1 mm) made of amorphous selenium, and the other is a novel hybrid panel detector using an x-ray photoconductor made of polycrystalline Cd(Zn)Te. As a result, excellent resolution and good linear sensitivity have been achieved. These detectors have great promise as next-generation digital imaging systems for fluoroscopy and radiography. Keywords: amorphous selenium (a-Se), cadmium telluride (CdTe), Cd(Zn)Te, flat-panel detectors, flat-panel displays, medical diagnostics, thin-film transistors, x-ray imaging.
electrical signals. And compared with II-TV systems, a significantly thinner form can be achieved that enables highresolution x-ray images to be obtained over a large area with no geometric distortion. Consequently, these new imaging systems have the potential to supersede traditional x-ray systems. The flat-panel x-ray image detectors can be classified into indirect-conversion and direct-conversion types that are differentiated by the x-ray detection principle used. The indirect-conversion type first converts x-rays to visible light in a phosphor layer (e.g., a screen such as Gd2O2STb or a structured scintillator such as CsITl) and then converts the visible light into electrical signals in photodiodes.3–5 The directconversion type converts x-rays directly to electrical signals in an x-ray conversion layer made of an x-ray photoconductor, for which amorphous selenium (a-Se) is the most commonly used material.6–8 These two concepts are compared in Figure 1. In general, although the indirect-conversion type can take advantage of the excellent photodetection capabilities of photodiodes, it suffers from inherent deterioration of spatial resolution due to optical scattering. It is also a complicated process to build a
Introduction In the field of medical diagnostics, a variety of imaging systems have traditionally been used to generate x-ray images, including screen/film (S/F) systems utilizing a phosphor screen and film, computed radiography (CR) systems using laser scanning to capture a latent image recorded on an imaging plate, and image-intensifier television (II-TV) systems combining a photomultiplier tube with a CCD (chargecoupled device). Recently, however, the rapid growth of digital image processing technologies and computer network systems has brought about the development of new digital imaging systems called flatpanel x-ray image detectors that use solidstate x-ray imagers.1,2 The key technology permitting this advance is a large-area amorphous silicon thin-film transist
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