Position emission tachograph - by determn. of number of base noise signals due to chance coincidences, subtracted from total coincidences detected (SE 25.5.81)

摘要

An appts. for carrying out positron emission tachography, i.e. the formation of visible images along one or several planes passing through the body of a patient, incorporates a source of radiations, a number of detectors, first and second coincidence devices and a difference-forming device. The source of radiations directs positrons into the zone being studied. The detectors detect the photons emitted when a positron is annihilated in this area. The first coincidence device determines the number of coincidences in time between photons detected by detectors placed at 180 deg. to one another. The second coincidence device determines simultaneously the number of coincidences in time between photons detected by individual detectors and photons detected with a time lag by detectors placed at 180 deg. The difference-forming device forms the difference between the number of coincidences in time determined by the first coincidence device and the number of coincidences in time detected by the second coincidence device. The difference is a measure of the number of positron annihilations in the zone. The appts. gives a very precise determn. of the number of base noise signals due to chance coincidences and subtracts this base noise from the total number of coincidences detected. Dead time is eliminated.