How Images Are Obtained

Radiographs are images that are obtained using ionizing radiation. An X-ray passes through a patient to hit a specialized detector.  Historically, images were created and printed on film. Currently, radiography has become digitalized and the images are viewed on specialized computer workstations.

X-rays are discrete quantities of electromagnetic radiation that are produced outside the nucleus of an atom. X-rays are produced in a generator by the interaction of an electron beam with a Tungsten target. The X-rays are emitted from the generator as a beam and travel through a patient.  Tissues absorb (attenuate) some of the x-rays, while others pass through and hit the cassette.  The attenuation properties vary among different tissue types, resulting in a heterogeneous distribution of X-rays emerging from the patient to hit the detector (historically called a cassette).

Screen-film radiography:  The cassette contains film and intensifying screens. When the X-rays hit the film, a photochemical interaction occurs, causing the metallic silver in the film to precipitate. This renders the film black when it is developed chemically.
Digital (computed) radiography: The cassette contains a photo-stimulable phosphor detector system.  When the X-rays hit the cassette, a fraction of the absorbed energy is trapped in the detector.  A red laser light stimulates the emission of the trapped energy, and blue-green light is emitted, collected, and converted into an electric signal.  This is digitized and stored as a pixel value, with the gray scale dependent upon the amount of energy absorbed & released.