The abdominal organs are surrounded by mesentery and omentum. On radiographs, the fat stored in these structures are what allow us to see the serosal surfaces of the soft tissue density organs. Fat is hyperechoic on ultrasound, giving a fairly uniform background for the abdominal organs. You can see examples in the falciform fat ventral to the liver, and retroperitoneal fat surrounding the kidneys. Sometimes the fat is even more hyperechoic than usual, and that can be an important clue in diagnosing disease.
Fat is hyperechoic when there is a diffuse effusion in the peritoneal cavity. Dogs and cats with ascites or hemoabdomen will have mesentery that is uniformly hyperechoic compared to normal. You can compare it to an unaffected area such as retroperitoneal or subcutanous fat to confirm.
The mesentery and omentum can also be focally hyperechoic. This is often caused by a small, localized effusion or inflammation of the peritoneum. The important thing to look for is what is the origin of the effusion or the peritonitis. It may be a bowel mass with compromised wall or outright rupture, or it may be pancreatitis. The localized inflammation or effusion is related to localized disease. Other examples include splenic torsion, colitis and biliary mucocele.
The retroperitoneal fat can also appear focally hyperechoic. When animals have acute renal disease such as pyelonephritis or an obstruction, there may be a hyperechoic halo of fat surrounding the affected kidney or kidneys. This is a particularly useful sign in cats when both kidneys are abnormal, and you are trying to decide which is affected by the acute epsisode of disease. Depending on the severity of disease, it may then extend to free fluid surrounding the kidneys or in the septae of fat in the retroperitoneum.
Expect to see hyperechoic fat when:
- there is diffuse peritoneal effusion
- there is a small amount of effusion
- there is diffuse or focal peritonitis
- the organ central to the focal hyperechoic fat is inflamed or ruptured