This 36-year-old patient with protruding abdomen has fluid collection in the omental bursa (lesser sac) on CT imaging. The omental bursa (lesser sac) is the cavity in the abdomen that is formed by the lesser and greater omentum. It is connected with the greater sac via the omental foramen.
The omental bursa is a complex and important anatomical region. It serves as a barrier to block pathological processes, but it is also a channel for disease spread in the abdominal cavity. It is a large recess of the peritoneal cavity formed by a double-layered fold of serous peritoneum situated inferiorly to the liver, posteriorly to the lesser omentum and the stomach and anteriorly to the pancreas.
Under physiological conditions the omental bursa is poorly visualized, both because it is situated deeply in the abdomen and because the cavity is merely a potential one containing only a small amount of liquid, which acts as a lubricant. However, the omental bursa may become clearly visible in the presence of disease.
The pathological conditions of the omentum are varied, and findings may range from fluid collection to widespread infiltration. Ascites (regardless of the cause), pancreatic necrosis during and after acute pancreatitis, chronic pancreatitis, surgery involving the pancreas or trauma may cause peripancreatic fluid collections. The omental bursa may therefore present noninflammatory or inflammatory fluid collections in different phases, pancreatic pseudocysts and hematoma.
a link to anatomy https://eidimages.s3.us-west-1.amazonaws.com/images/coronal%20diagram%20of%20abdomen.png-1592945646758.png
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The arrowed fluid is contained in a space behind the stomach but in front of the retroperitoneal structures (e.g. the pancreas), i.e. the lesser sac.