Observations on CT differentiation of pleural and peritoneal fluid

Distinguishing juxtadiaphragmatic pleural and peritoneal fluid by computed tomography (CT) is sometimes difficult. We evaluated the distribution of fluid in 50 consecutive patients with pleural effusion or ascites or both. Juxtadiaphragmatic pleural fluid was nearly always found posteromedially, whereas peritoneal fluid typically distributed lateral to the liver or spleen or both in the posterior upper abdomen. This difference in distribution is easily explained by pertinent upper abdominal peritoneal and extraperitoneal anatomy. On the basis of distribution alone, pleural and peritoneal fluid could be differentiated in 47 of 50 patients. Difficulty was encountered with loculated pleural fluid and with ascites having a large left subphrenic component; in those situations, previously reported CT signs may be helpful.