What are the radiological and ultrasound findings of peritoneal effusion?

Radiologically, the characteristics of effusion are loss of detail of the serosas, separation between the intestinal loops, and abdominal distension if the volume is considerable.
In ultrasound, the fluid is observed as anechoic when it is not very cellular and increases in echogenicity as the cellularity of the fluid increases. The retroperitoneal effusion is diagnosed by the presence of fluid around the kidneys without the presence of free fluid in other abdominal locations.