The Annihilation Coincidence Method of Localizing Positron-emitting Isotopes, and a Comparison with Parallel Counting

The localization of positron-emitting radioisotopes by the detection of annihilation coincidences has been studied, with particular reference to the use of oxygen-15 in the study of lung function. High coincidence counting rates are needed, and it is important to minimize and then correct for the random coincidences which would otherwise spoil the spatial resolution, and falsify the measurements of the steady fall of oxygen-15 activity in the lungs during breath-holding. (This is one of the observations made in the oxygen-15 test.) The problem is accentuated by the presence of large activities in the lung surrounding the region under investigation, especially if the ventilation of this selected region is defective.