n. a device, used in diagnostic radiology or radiotherapy, to produce a narrow beam of radiation by means of metallic sheets, acting like a diaphragm in a camera, that control the size of the beam from a radiation source. Many newer *linear accelerators use multi-leaf collimators, a specialized form of collimator using individual ‘leaves’ (1 cm or smaller) to shape the radiation *treatment field around the tumour. Collimators are also used on radiation detectors, in particular in *gamma cameras, for which the exact source of radioactivity needs to be known to produce an accurate image.
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