While the term digital radiography started being used in the mid-1980s, digital imaging became more developed in the 1990s. Computed Radiography (or CR) uses an analog screen-film detector which combines cassette-based Photostimulable Storage Phosphor (PSP) and plate reader. The phosphor absorbs x-ray energy that passes through the patient and stores the x-ray latent image as a two-dimensional array of electrons. After the latent image is transferred to a computer, the plate can be cleared by placing it in bright light.
On the other hand, direct digital radiography (DR) uses x-ray sensitive plates to directly capture data during patient examination, and then transferring the data to a computer system without a cassette. These plates are more commonly referred to as Flat Panel Detectors (FPDs), which use a combination of amorphous silicon detectors with Cesium Iodide (CsI) scintillators that convert x-ray to light, which is translated by thin film transistors (TFT) into digital data on a viewing screen.
There are two types of flat panel detectors: indirect and direct conversion detectors. With indirect conversion, the outermost layer is a phosphor screen (commonly CsI); the detector, based on amorphous silicon photodiode. Indirect panels convert x-rays to light and then to a charge. Direct conversion flat panel detectors’ outermost layer is a high voltage bias electrode and the detector is based on amorphous selenium (not amorphous silicon). These panels convert x-rays directly into a charge. Indirect conversion also occurs CCDs (Charge Coupled Device), which consist of scintillator and lens.
There are inherent differences between DR and CR in the areas of brightness, latitude, contrast, and edge enhancement.
Brightness is the balance of light and dark shares in a displayed image. High brightness, also referred to as density or intensity, settings produce bright images and low settings produce dark images. The contrast determines how many gray levels are displayed and the density determines intensity.
The latitude, or dynamic range, is the range of receptor exposures over which an image and contrast will be formed.
Contrast is the form of differences in optical density values between various points within the image, such as between subject and the surrounding background.
Edge enhancement is an image processing filter that enhances the edge contrast of an image in an attempt to improve its sharpness.