Acute Pancreatitis

Acute pancreatitis is generally diagnosed clinically. The role of imaging is to clarify the diagnosis when the clinical picture is confusing, to assess severity, to determine prognosis, and to detect complications. Inflammation of the pancreatic tissue leads to disruption of small pancreatic ducts, resulting in leakage of pancreatic secretions. Because the pancreas lacks a capsule, the pancreatic juices have ready access to surrounding tissues. Pancreatic enzymes digest fascial layers, spreading the inflammatory process to multiple anatomic compartments.

Causes of acute pancreatitis

1. Alcohol abuse (most common cause of chronic pancreatitis)
2. Gallstone passage/impaction (most common cause of acute pancreatitis)
3. Metabolic disorders - Hereditary pancreatitis (autosomal dominant), Hypercalcemia, Hyperlipidemia (types 1 and 5), Malnutrition
4. Trauma - Blunt abdominal trauma, Surgery, Endoscopic retrograde cholangiopancreatography, Penetrating ulcer
5. Malignancy - Pancreatic adenocarcinoma, Lymphoma, Drugs -(steroids, tetracycline, furosemide, many others)
6. Infection - Viral (mumps, hepatitis, infectious mononucleosis, AIDS), Parasites (Ascariasis, Clonorchis)
7. Structural - Choledochocele, Pancreas divisum
8. Idiopathic (20% of cases of acute pancreatitis)

Imaging studies of acute pancreatitis may be normal in mild cases. Contrast-enhanced MDCT provides the most comprehensive initial assessment; however, US is useful for follow-up of specific abnormalities, such as fluid collections. Abnormalities that may be seen in the pancreas include: (1) focal or diffuse parenchymal enlargement, (2) changes in density because of edema, and (3) indistinctness of the margins of the gland owing to inflammation.

Abnormalities in the peripancreatic tissues include stranding densities in the fat with indistinctness of the fat planes and thickening of affected fascial planes.

Complications

1. Pancreatic fluid collections (collections of enzyme-rich pancreatic juice) - Acute: resolve spontaneously in 50% of cases; may be intrapancreatic, anterior pararenal space, lesser sac, or extend anywhere in the abdomen, into solid organs, or even into the chest.
2. Pseudocyst - round or oval, encapsulated pancreatic fluid collection encased by a distinct fibrous capsule; require at least 4 weeks to develop; about 50% will spontaneously resolve, whereas the remainder will require catheter or surgical drainage.
3. Liquefactive necrosis of pancreatic parenchyma - seen as lack of parenchymal enhancement during bolus contrast administration on CT, often multifocal. Morbidity and mortality increase dramatically when necrosis is present.
4. Infected necrosis (bacterial infection in necrotic tissue) - Seen as an area of nonenhancing pancreatic tissue containing gas. Confirmed with needle aspiration. Infected necrosis generally requires surgical drainage.
5. Abscess (circumscribed collection of pus in area with little or no necrosis tissues) - seen as a fluid collection with a thick wall. Effectively treated with catheter drainage.
6. Hemorrhage (resulting from erosion of blood vessels and tissue necrosis) - CT shows high-attenuation blood in the retroperitoneum.
7. Pancreatic ascites (leakage of pancreatic secretions into peritoneal cavity).
8. Pseudoaneurysm - autodigestion of arterial walls by pancreatic enzymes results in pulsatile mass that is lined by fibrous tissue and maintains communication with parent artery.
9. Splenic Vein Thrombosis - Although more common in chronic pancreatitis, it can be seen in focal or diffuse acute pancreatitis.

US- or CT-directed aspiration biopsy may be needed to confirm the presence of pancreatic abscess. Image-directed catheter placement is an alternative to surgical drainage of pancreatic fluid collections. Contrast-enhanced MR is equivalent to CT in the assessment of pancreatitis.