Physics, Techniques and Procedures

Coincidence counting

method of counting employing a coincidence circuit so that an event is recorded only if events are detected in two sensing devices simultaneously. Such counting methods may be used to reduce background noise if a radioisotope emits more than one detectable radiation event in coincidence. The effective background from sources other than this radioisotope can be greatly reduced through the use of coincidence counting. The requirement for a coincidence between two detectors eliminates background counts that occur in only one detector at a time. Coincidence detection of gamma rays is used in PET imaging. When the two gamma rays originating from a single positron-electron annihilation reaction are detected within a specific time window, the annihilation is described as a coincident event. Coincident events identify the presence of the positron-emitting labelled compound. The ability to distinguish true coincidence counts from scattered and random coincidence counts is the basis of the signal in PET imaging.

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