The displaced crus: a sign for distinguishing between pleural fluid and ascites on computed tomography

Fluid collections in the lower pleural cavity and upper abdominal cavity may sometimes appear similar on computed tomography. Lateral displacement of the diaphragmatic crus and interposition of fluid between the crus and vertebral column indicate an intrapleural component. Demonstration of fluid anterior to the crus establishes an intraperitoneal component. Medial location of the posterior recess of the fluid collection and the intercostal bulging of its lateral margin suggest a pleural effusion.