2–10
12/12/94 – 1234563
Section 2: Computed Radiography (CR) Theory
From X-ray to CR
In general X-rays are used in medical imaging to produce an image of specific anatomical features on a surface, which can be interpreted by a radiologist or other
medical personnel. There are three basic phases to both conventional X-ray systems (screen/film) and computed radiography such as the CR 800.
Producing the Aerial Image
In both conventional and computed radiographic systems, X-rays are produced in an X-ray tube and directed toward an image receptor. The X-rays interact with
everything in their path. When the X-rays encounter the body of the patient, some penetrate the patient and some do not. The result is an aerial image with varying
degrees of X-ray power (varying numbers of X-ray photons).
Creating the Latent Image
When the image receptor is exposed to the X-rays in the aerial image a “latent” image is formed on the receptor. In conventional screen/film systems, the image is
captured on sensitized radiographic film. In computed radiographic systems, such as the CR 800 System, the X-ray photons that reach the storage phosphor screen
charge the phosphor, creating a latent image on the Screen.
The latent image is made from the X-rays that penetrate the patient; however, the rays that do not emerge from the patient, but are absorbed, also contribute to the
image, providing the lighter areas of the captured image.
Capturing, Converting, and Archiving the Visible Image
Finally, the latent image must be converted into a visible image, which can be interpreted by the radiologist, transported from one place to another, and stored or
archived for later use. In conventional systems, the radiographic film is processed through chemicals and the image is fixed onto the film.
In computed radiography, the latent image on the storage phosphor plate is scanned by a laser beam, which stimulates the charged phosphors on the plate. Blue light
is emitted from the stimulated phosphors, collected, and converted into analog electrical signals. The image is then converted into digital signals and processed, thus
making it available to be stored and displayed by a computer system and routed to other computers and printers through a network.
