Computed Radiography (CR) is a cassette-based system that uses a special solid-state detector plate instead of a film inside a cassette. The exterior dimensions and appearance of the computed radiography cassette are the same as those of a conventional film cassette. The CR cassette is inserted into the bucky tray followed by exposure in accordance with a conventional film cassette. The contrast resolution of computed radiography is of higher quality than both conventional film and screen systems. This is because the CR cassette contains a photostimulable storage phosphor imaging plate that is sensitive to radiation. When the X-rays strikes the solid-state plate, energy is trapped and the CR cassette records the digital images and can be used by the radiologist. These CR cassettes can be used numerous times and are applied to several imaging modalities such as computed radiography, computed tomography, ultrasound, digital radiography, fluoroscopy, mammography, magnetic resonance and nuclear medicine. The main advantage of digital images is that the contrast and speed are not fixed during processing compared to film images. Computed radiography has been used over 30 years in the clinical setting and is the most established of the digital radiography technologies. However, the limitations of CR systems include cassette handling, low detective quantum efficiency and long readout of plates have been addressed by other technological advances. These include, automated image data acquisition computed radiography systems which can produce a readout in less than 10 seconds. This technology uses a more advanced photostimulable storage phosphor imaging plate which consists of a needle-shaped phosphor caesium bromide crystal. These crystals reduce the light diffusion because the needle acts as a light guide and the phosphors have increased detective quantum efficiency. Other technologies are based on digital radiography (DR) and the images are recorded on an electronically readable device linked directly to a computer processing unit. These DR systems are made up of an array of small solid-state detectors and sensors and function by converting incident X-ray photons to form the digital image without any cassette.
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