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central ray (CR)
the portion of an x-ray beam that is directed toward the center of the film or of the object being radiographed.
The theoretical center of an x-ray beam. The term designates the direction of the x-ray photons as projected from the focal spot of the x-ray tube to the radiographical film.
See also: ray
the center of the area of radiation created as the x-ray beam diverges. The further from the center of the ray one is, the more distortion there is and, if the beam is not correctly centered over the part to be viewed, there will be more distortion still. The central point is the point of minimum distortion.