The sonographic appearances of pleural fluid.

In order to document the sonographic changes of pleural fluid the appearances of 93 presumed pleural fluid collections in 89 patients were recorded. In all patients the clinical or radiographic appearances were atypical and they were referred for guided aspiration or drainage of suspected pleural fluid. Ultrasound guided aspiration was successful in 90 collections in 86 patients. It was unsuccessful in three "collections" subsequently shown or presumed to be of a solid nature. In nine of the proven fluid collections an earlier attempt at blind aspiration had been unsuccessful. 54 (60%) of the proven fluid collections had a characteristic sonographic appearance of pleural fluid as an anechoic space unrestricted within the pleura. Of the remaining 36 collections, 16 had diffuse internal echoes, 12 were confined to one area of the pleural space (primary loculation) and 13 had internal septa (secondary loculation). Dynamic signs were demonstrated in 75 (83%) of the collections as flapping movements of atelectatic lung (51), of internal septa (12), or of debris attached to the wall of the collection (25). Three (4%) of fluid collections had sonographic features indistinguishable from those of solid pleural lesions, indicating that in a small percentage of cases sonography will not reliably distinguish between solid and fluid in the pleural space.