Current advances and future trends in X-ray digital detectors for medical applications

Despite the progress made in imaging modalities based on nonionizing radiation such as ultrasound and magnetic resonance imaging, X-ray is still the primary imaging method in most radiology departments. In recent years, digital electronics and manufacturing methods have led to many approaches for the design and construction of X-ray detectors. The ideal medical X-ray system is a digital, flat-panel able to perform all current radiological modalities at reduced dose. It would, immediately after the patient's X-ray exposure, provide a high quality radiograph on a video monitor. It would also be usable for real-time imaging (e.g., digital cine loops and fluoroscopy). The physical form would be similar to a film/screen cassette (except for the addition of an umbilical cord) which could easily fit into the Bucky tray of existing X-ray rooms. There are two main approaches both based on active matrix technology developed for lap-top displays. The first, called the direct method employs a photoconductive layer of amorphous selenium (a-Se) to detect X-rays. The charge image formed on the surface of the a-Se is digitally read out in situ using a two-dimensional array of thin-film transistors (TFT's), i.e., an active matrix. The second flat panel method is called the indirect method and uses an active matrix to readout a phosphor layer. Phosphors give off light on interaction with X-rays and this is sensed by photo diodes on the active matrix. The relative practicality of the direct and indirect active matrix readout systems is an area of current investigation.