Sometimes, pleural effusion may be noticed incidentally on abdominal scans. Below image shows transverse section of the liver with anechoic area in the posterior aspect, which corresponds to right lung/pleural area; black = fluid = pleural effusion. The bright line encircling the liver separating it from the fluid represents diaphragm.

Note the anatomic correlates – the ultrasound beam is actually oblique in between the two transverse sections shown. In such cases, go up and scan in the coronal plane to confirm your findings.

Below is a nice example showing the right pleural effusion in both transverse and coronal planes. Also note small amount of ascites in between the diaphragm and liver tissue.