The revised Atlanta classification of acute pancreatitis considers two overlapping phases with two peaks of mortality. The early phase usually last for the first week and the primary source of mortality is the systemic inflammatory response syndrome (SIRS) and multiple organ failure (MOF). In the late phase, which may last for weeks or months, local complications can (apart from persistent systemic inflammation) develop.

1) Interstitial edematous pancreatitis

Acute inflammation of the pancreatic parenchyma and peripancreatic tissues, without detectable tissue necrosis.

• Homogeneous or slightly heterogeneous (due to edema) enhancement of pancreatic parenchyma
• Localized or diffuse enlargement of the pancreas
• Normal appearance or mistiness and stranding of peripancreatic soft tissue

2) Necrotizing pancreatitis

Three forms of acute necrotizing pancreatitis can be distinguished according to the revised Atlanta classification. Necrosis can be either sterile or infected.

a) Pancreatic parenchymal necrosis alone:

Homogeneous nonenhancing areas of variable attenuation that become later more heterogeneous. The extent of parenchymal necrosis can be cathegorized as <30%, 30 - 50%, >50%, or more recenty only <30% and >30%.

b) Peripancreatic necrosis alone:

Contrast enhanced CT shows heterogeneous nonenhancing areas without detectable liquefied component, usually localised in the retroperitoneum or the lessec sac.

c) Pancreatic parenchymal necrosis with peripancreatic necrosis

This is a combination of a) and b) and it is the most common type.

3) Pancreatic and peripancreatic collections

The revised Atlanta classification distinguishes fluid and non-liquefied collections.

a) Acute peripancreatic flud collections - APFC

Occur in patients with interstitial pancreatitis during the first four weeks. They have no wall and usually outline retroperitoneal fascias, especially the anterior renal fascia (Gerota's fascia).

b) Pseudocyst

In patients with interstitial pancreatitis, acute peripancreatic fluid collections (APFCs) may within 4 weeks gradually became walled off and form a pseudocyst with homogeneously low density content surrounded by an enhancing wall.

c) Acute necrotic collection - ANC

Acute necrotic collection describes persistent necrotic collection within the first 4 weeks of necrotizing pancreatitis. It contains both fluid and necrotic material. It has a complex structure on contrast-enhanced CT, however initially, in the first week it may also appear homogeneous. It can be either sterile or infected.

d) Walled-off necrosis, WON

After 4 weeks, an acute necrotic collection gradually develops a wall that surrounds liquefied and non-liquefied content. It can be either sterile or infected.