Factors involved in the detection of pulmonary emboli by labelling techniques

Experiments were set up to assess quantitatively the problem of detecting, by scanning and gamma-camera, labelled pulmonary emboli in the presence of background radioactivity. In vitro, a relative concentration of radioactivity of 2:1 was required to detect a 1-1 cm diameter "hot spot". The results in vivo suggest that the level of radioactivity of a pulmonary embolus situated in the centre of the lung field should be at least six times that of the circulating blood.