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Physics, Techniques and Procedures

Automatic exposure control

a device (also called autotimer or phototimer) used to control the length of an X-ray exposure. These devices use a signal, acquired at the exit side of the patient or object to control the length of the exposure. Normally, the automatic exposure control system uses detectors placed in front of the film, but some systems utilize detectors placed behind the film/cassette. Most often, three detectors are used, mounted in a pattern as shown in Fig.1. Each detector is normally an ionization chamber or a very thin layer of scintillating material mounted on a light-guiding plate (plexiglass). The signal from the ionization chamber is amplified and is used to control the exposure. When irradiated, the scintillating material produces light which is guided through the plexiglass plate and illuminates a photomultiplier tube. This converts the signal into an electric current which is amplified. Prior to use, both systems have to be calibrated in order to produce an adequate density on the film.The most appropriate system is dependent on the clinical purposes of the equipment. Ionization chambers generally have an energy response (variation in sensitivity with kVp) which does not match the sensitivity of the intensifying screens of phosphor plates (see photostimulable phosphor plate). However, for systems that are used inside a narrow kVp interval, this is not a major problem, especially since such a system is stable and rugged. The scintillation detectors can be better adapted to the spectral sensitivity variation of the intensifying screens of phosphor plates, especially if the scintillating material is identical to the screen or phosphor material. This type of detector is, however, not as robust as the ionization chamber system. Other systems for automatic exposure control have been suggested and are based on scintillating material (the same as used in the cassette screens) from which the light emitted is sampled in a photodiode or a phototransistor.

MN

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Fig.1

The arrangement of three detectors (1-3) in front of a film is shown.
Automatic exposure control, Fig.1