Role of CT and US in assessing the delayed complications of pancreatitis
A. Ten percent to 20% of patients with acute pancreatitis and fluid collections develop pseudocysts after 4 to 6 weeks. Most pseudocysts smaller than 5 cm in diameter regress spontaneously. Drainage may be indicated for pseudocysts (1) failing to resolve after 6 weeks; (2) remaining larger than 5 cm in diameter; or (3) causing pain, infection, hemorrhage, bowel obstruction, or fistula.
US: Pseudocysts appear as anechoic fluid collections with or without internal debris surrounded by a thin wall. May appear complex or even solid because of the debris.
CECT: Pseudocysts appear as well-defined fluid collections with a uniformly thin, enhancing wall. Gas bubbles inside a pseudocyst relate to infection or enteric fistula formation.
B. Acute peritonitis may occur if a pseudocyst ruptures into the peritoneal cavity.
C. Necrosis is diagnosed by a lack of contrast enhancement within the pancreatic tissue. It is best demonstrated by MDCT in the PVP with an accuracy of 85%. CT evidence of necrosis correlated to morbidity/mortality is as follows:
• No necrosis: mortality rate (0%) and morbidity rate (6%)
• Mild (< 30% of the total gland) necrosis: mortality rate (0%) and morbidity rate (≈ 50%)
• Severe (> 50% of the total gland) necrosis: mortality rate (11%-25%) and morbidity rate (75%-100%)If secondarily infected, gas may be present in the area of necrosis (i.e., emphysematous pancreatitis). Infected areas usually do not contain gas, and a percutaneous aspirate is needed to confirm the diagnosis and identify the organism.
D. Abscesses result from liquefactive necrosis with subsequent infection and usually occur 4 weeks after the onset of acute pancreatitis. Rate of abscess formation varies with the amount of necrosis.
US: Abscesses appear as a hypo- to anechoic masses, sometimes containing hyperechoic gas, surrounded by a thickened wall.
MDCT: Abscesses appear as focal low-attenuation fluid collections with thick enhancing walls. If gas is present, an abscess needs to be excluded. The distinction between abscess and infected necrosis is difficult, but important because a pancreatic abscess often requires more aggressive treatment.
E. Enzymatic breakdown of the arterial wall can result in a pseudoaneurysm, most commonly in the (1) splenic, (2) gastroduodenal, or (3) pancreaticoduodenal arteries. Up to 10% of pseudoaneurysms rupture, usually into a pseudocyst, but occasionally into the retroperitoneum, peritoneum, pancreatic duct, or bowel. This results in massive hemorrhage.
US: Color Doppler US is sensitive in detecting pseudoaneurysms and their complications.
MDCT: MDCT is best at identifying pseudoaneurysms, which usually present as densely enhancing structures in close proximity to a pseudocyst.
F. Splenic vein thrombosis increases the risk of bleeding gastric varices. It is detected by absence of enhancement in the expected region of the splenic vein on MDCT in the PVP. It is present in up to 45% of cases of chronic pancreatitis. Color Doppler can also make the diagnosis.