Atelectasis and subpulmonic fluid: a CT pitfall in distinguishing pleural from peritoneal fluid

The ability to distinguish between pleural and peritoneal fluid is important in the management of patients with peridiaphragmatic fluid collections. The differentiation of subphrenic fluid from pleural effusion by CT may be difficult and requires a detailed understanding of the normal appearance of the diaphragm on transaxial images. We report 12 patients in whom peridiaphragmatic fluid collections could potentially be misinterpreted as peritoneal rather than pleural fluid. In all cases the presence of subsegmental, basilar atelectasis with subpulmonic fluid created confusing CT images. Subsegmental atelectasis formed a curvilinear band at the lung base that simulated the hemidiaphragm. Fluid anterior to the atelectasis caused a false impression of subdiaphragmatic (peritoneal) fluid. Computed tomography through the upper abdomen excluded intraabdominal fluid in all cases. In four cases ultrasonography confirmed fluid in a supradiaphragmatic, subpulmonic location, excluded peritoneal fluid, and identified subsegmental atelectasis. The presence of subpulmonic fluid and subsegmental atelectasis requires meticulous interpretation of transaxial CT images to avoid the incorrect localization of peridiaphragmatic fluid collections.