The interface sign

摘要

Ascites and pleural effusions represent abnormal accumulations of fluid in the peritoneal and intrathoracic cavity, respectively and are common findings on chest and abdominal computed tomography (CT). These conditions sometimes share the same etiologies, sometimes even coexisting in the same patient. When they occur separately, their differentiation by CT images is not always straightforward, as free fluid tends to accumulate in posterior and lateral pleural recesses and around the liver and spleen in patients lying supine [1]. However, there are some classic CT findings that aid in correct characterization [1, 2]. One classic sign described for this differentiation is the interface sign: in ascites, there is a direct contact of the free peritoneal fluid with the liver and/or spleen, leading to a sharp interface between the fluid and these organs. In pleural effusion, however, both liver and spleen are separated from the fluid in the thorax by the diaphragmatic crus, generating an ill-defined interface. It is noteworthy, however, that although helpful, the interface sign should not be considered isolated, as it may sometimes be misleading [1, 2]. This sign was able to correctly classify 80% of these two fluid collections in its original series [2] (Figs. 1 and 2).