Updated imaging nomenclature for acute pancreatitis.

Key points. 1. CT is used to confirm the diagnosis of acute pancreatitis when the diagnosis is in doubt and to differentiate acute interstitial pancreatitis from necrotizing pancreatitis, which is a key element of the updated Atlanta nomenclature. The acute interstitial variety accounts for 90-95% of cases, with acute necrotizing pancreatitis accounting for the remaining cases. 2. Necrosis due to acute pancreatitis is best assessed on IV contrast-enhanced CT performed 40 seconds after injection. Peripancreatic necrosis is a subtype of necrotizing pancreatitis in which tissue death occurs in peripancreatic tissues. This is seen in isolation in 20% of patients with necrotizing pancreatitis. 3. Simple fluid collections associated with acute interstitial pancreatitis are subdivided chronologically. A collection observed within approximately 4 weeks of acute pancreatitis onset is termed an "acute peripancreatic fluid collection (APFC)." A collection older than 4 weeks should have a thin wall and is termed a "pseudocyst." Both APFCs and pseudocysts can be infected or sterile. 4. Fluid collections associated with necrotizing pancreatitis are labeled on the basis of age and the presence of a capsule. Within 4 weeks of acute pancreatitis onset, a fluid collection associated with necrotizing pancreatitis is termed an "acute necrotic collection (ANC)" whereas an older collection is termed an area of "walled-off necrosis (WON)" if it has a perceptible wall on CT. The term "pseudocyst" is not used in the setting of necrotizing pancreatitis collections. Although an ANC and a (WON can be infected or sterile, infection is far more likely compared with acute interstitial pancreatitis collections. 5. The severity of acute pancreatitis is graded on the basis of the presence of acute complications or organ failure. Mild acute pancreatitis has neither acute complications nor organ failure. Moderate-severity acute pancreatitis is associated with acute complications or organ failure lasting fewer than 48 hours. Severe acute pancreatitis is characterized by single- or multiorgan failure persisting for greater than 48 hours.