Teaching Points-Differentiation between pleural and ascitic fluid

Differentiation between pleural and ascitic fluid on CT scans is sometimes a problem, and may be resolved by a number of signs, which are describe below. This portion from Suttons is very often asked to residents and is a very important exam question as well.

1. Displaced crus sign:  pleural fluid may collect posterior to the diaphragmatic crux and therefore displace the crus anteriorfy, whereas ascites collects anterior to the crus and may cause posterior

displacement.

2. Diaphragm sign:  As an extension of the displaced crus sign, any fluid that is on the exterior of the dome of the diaphragm is in the pleura. whereas any that is within the dome is aseites

3. Interface sign:  The interface between the liver or spleen and pleural fluid is said to be less sharp than that between the liver or spleen and ascites 

4. Bare area sign: The peritoneal coronary ligament prevents Such defects are the result of lung pathology, trauma or deliberate ascitic fluid from extending over the entire posterior surface of the liver, whereas in a free pleural space, pleural fluid may extend overthe entire posterior costophrenic recess behind the liver