Fig. 4. Semilog plot of tracer activity versus  time following inhalational delivery of tracer during (A ) control conditions and (B ) after pulmonary embolism (PE). For each condition, curves are shown separately for perfused and embolized regions of interest (ROIs). (A ) During control conditions, tracer washouts are linear and equivalent in regions that would become perfused and embolized after PE. (B ) After PE, the curve for perfused ROIs shows an increased slope at the beginning of the washout phase as compared with the same regions during control conditions (dotted line). Tracer washout from embolized ROIs is reduced as compared with the same regions before PE and also to the regions that remained perfused after PE. Evident nonlinearity is observed in both perfused and embolized ROIs.