Kinetics of the intrapleural distribution of a radioactive bolus

Through the use of a gamma-camera placed horizontally over the chest of 5 supine, anesthetized and spontaneously breathing dogs we studied the kinetics of the intrapleural spreading of 1 ml saline containing 2 mg of 99mTc labelled albumin (250 microCi/kg) injected at various points on the costal side. From the postinjection sequence of gamma-camera images, we defined, along the intrapleural spreading pathways, some regions of interest (ROI) displaying a preferential label accumulation. In all the animals the ROI roughly corresponded to the apical, mediastinal and latero-diaphragmatic regions. The kinetics of the intrapleural label distribution, estimated from the time course of the activity under the ROI, displayed an initial rapid phase, lasting a few minutes, followed by a marked slowing down of the exchange processes. At 30 min regional counts reached an apparent steady state, the activity values greatly varying (even by 10 fold) among the animals, for the ROI corresponding to the same anatomical region, independently on the site of injection. The mathematical modelling of the regional activity curves required the assumption of a progressive decrease in the exchange coefficients to account for a marked slowing down of the exchange processes with increasing time.