Pancreatic/peripancreatic fluid collections occur due to rupture of an obstructed pancreatic duct. The pancreatic juice that escapes may collect within the pancreas itself; however, due to the absence of a pancreatic capsule, it usually escapes into the adjacent tissues especially into the area of the lesser sac. The fluid does not usually escape into the greater peritoneal space as the foramen of Winslow becomes blocked by the surrounding inflammatory process. With the passage of time, such fluid collections, if they persist, develop a fibrous wall and so become a pseudocyst. This escape of fluid from the gland is probably helpful because if the pancreas had a capsule which retained the fluid, the gland would digest itself. The contents are composed not only of pancreatic juice but also necrotic cellular debris and blood. These fluid collections often spontaneously resolve. When one persists, its wall matures (which usually takes about 6 weeks) and it is termed a pseudocyst. A true cyst of the pancreas is lined by epithelium, a pseudocyst by fibrous tissue. Only 10% of patients with acute pancreatitis develop a pseudocyst, which is usually single, round to oval in shape, and can be either small or quite large. Pseudocysts can be found anywhere within the abdomen and can dissect up into the mediastinum or into the small bowel mesentery or down the retroperitoneum into the pelvis. They can also dissect into the parenchyma of the liver, spleen, or kidney and mimic a cyst of these organs. Most are adjacent to, but not within, the pancreas.

A pancreatic pseudocyst can slowly, spontaneously resolve. Factors mitigating against spontaneous resolution include a cyst larger than 5-6 cm, increasing size on serial sonograms, and the presence of multiple cysts. Pseudocyst can also rapidly disappear by rupturing into the pancreatic duct or an adjacent portion of the gastrointestinal tract. Other than the common sites (colon, duodenum, pancreatic duct, stomach and esophagus) rupture can occur into the pleural cavity, peritoneum, bronchus, neck, aorta, splenic artery, and portal vein. Rupture occurs due to the combined effects of enzymatic digestion, pressure necrosis, and vascular ischemia. Rupture may or may not be a benign event. For example, rupture into the stomach is well tolerated, but rupture into the splenic artery will cause hemorrhage. Other complications of untreated pseudocysts include bowel obstruction and infection.

The success rate of surgery for pancreatic pseudocysts varies from 16 to 80%. They have been treated with a varying degree of success by percutaneous aspiration with or without external drainage. An alternative to percutaneous aspiration is percutaneous transgastric drainage with an indwelling catheter that is only removed when drainage has ceased and the fluid collection has disappeared on sonography or CT. Another approach is percutaneous insertion of a stent between a pseudocyst and the stomach when they abut each other. The stent is left in place and is removed endoscopically only when complete drainage of the pseudocyst has occurred as indicated by sonography or CT. If a surgical approach is being contemplated, a repeat sonogram should be performed immediately prior to operating to confirm that the pseudocyst is still present and has not spontaneously decompressed.

Pseudocysts of the head and body of the pancreas are usually easily seen sonographically. When they are located in the tail of the pancreas, they are more difficult to detect, especially when small. The success rate overall in detecting pseudocysts sonographically varies from 50 to 92%.

The wall of a pancreatic pseudocyst is generally well-defined and smooth; however, it can be irregular especially when hemorrhage is present. The contained fluid can be entirely echofree or have a varying amount of internal echoes or even a fluid-debris level. When the cyst wall is irregular and the contents are very echogenic, the possibility of superimposed hemorrhage or infection has to be considered. On occasion septations can occur within a pseudocyst, thus mimicking a mucinous cystic neoplasm. This problem can usually be resolved by the clinical history and the presence of other sonographic findings of inflammatory pancreatic disease.

The CT number of the fluid in a pseudocyst usually is close to 0H, but, it can be as high as 25-30H. The wall is best identified using intravenous contrast and is usually smooth and thin (3-4 mm). A thick, irregular wall and/or greater than water attenuation may indicate the presence of hemorrhage or infection. Calcification in a pseudocyst wall is quite unusual. Overall, CT is better than sonography in detecting pseudocysts and their associated complications, especially those in locations remote from the pancreas. Sonography is certainly adequate for following known pseudocysts.