Imaging Characteristics Differentiating Chronic Pancreatitis from Autoimmune Pancreatitis
The key imaging distinction is that autoimmune pancreatitis demonstrates diffuse "sausage-shaped" pancreatic enlargement with homogeneous delayed enhancement and a capsule-like peripheral rim, while chronic pancreatitis shows progressive atrophy with calcifications, ductal dilation, and eventual gland shrinkage. 1, 2, 3
Autoimmune Pancreatitis: Characteristic Imaging Features
Pancreatic Parenchyma
- Diffuse enlargement creating a characteristic "sausage-shaped" appearance is the hallmark finding, occurring in approximately 95% of cases 3, 4
- Capsule-like peripheral rim surrounding the enlarged pancreas is visible on CT/MRI in approximately 30% of patients 4
- Homogeneous delayed enhancement on dynamic CT distinguishes AIP from pancreatic cancer 5
- Focal enlargement can occur in approximately 5% of cases, making differentiation from malignancy particularly challenging 3, 5
Pancreatic Duct Changes
- Multifocal narrowing of the main pancreatic duct without upstream dilation is characteristic, occurring in approximately 85% of cases 4
- The narrowed segments show few or no contrast-filled side branches in the area of stenosis 5
- Longer segments of stenosis (compared to the short, abrupt strictures of pancreatic cancer) 5
- Absence of calcifications in the pancreatic parenchyma distinguishes AIP from chronic pancreatitis 3
Biliary Involvement
- Tapered narrowing of the distal common bile duct occurs in approximately 75-90% of patients 4
- The narrowed bile duct segment often shows contrast enhancement on imaging 4
- This finding commonly presents as painless obstructive jaundice 2, 3
Vascular Changes
- Narrowing of peripancreatic veins (portal vein, splenic vein, superior mesenteric vein) occurs in approximately 70% of cases 4
- This finding is reversible with corticosteroid therapy 4
Chronic Pancreatitis: Characteristic Imaging Features
Pancreatic Parenchyma
- Progressive atrophy with normal tissue replaced by fibrous tissue 6
- Gland shrinkage in advanced stages, contrasting with the enlargement seen in AIP 6
- Significant calcification throughout the gland is a late finding specific to chronic pancreatitis 6
- Irregular, heterogeneous parenchymal texture rather than the homogeneous appearance of AIP 6
Pancreatic Duct Changes
- Main duct dilation with strictures develops as the disease progresses 6
- The duct becomes dilated and strictured, unlike the narrowed duct without upstream dilation in AIP 6
- Side branch abnormalities are common 6
Imaging Modality Selection
- CT scanning is the initial imaging of choice for advanced chronic pancreatitis with sensitivity of approximately 60% for USS 6
- MRI with MRCP is more sensitive and accurate for detecting chronic pancreatitis than CT, particularly for early disease 6
- EUS detects mild parenchymal and ductal abnormalities not visible on CT, with sensitivity of 68-100% and specificity of 78-97% 6
- MRCP with secretin increases diagnostic yield and correlates with pancreatic function tests 6
Critical Diagnostic Pitfalls
Focal AIP Mimicking Malignancy
- Focal AIP can present with identical features to pancreatic cancer: jaundice, weight loss, elevated CA19-9, and pancreatic masses 2, 3, 5
- Serum IgG4 >280 mg/dL is the most sensitive and specific marker for differentiating type 1 AIP from cancer 1, 2
- Lower CA19-9 levels favor AIP over malignancy 5
- Homogeneous delayed enhancement on dynamic CT favors AIP 5
Reversibility with Treatment
- All imaging abnormalities in AIP (parenchymal enlargement, duct narrowing, vascular narrowing, bile duct stenosis) are invariably reversible with corticosteroid therapy 4
- This reversibility is a key distinguishing feature from chronic pancreatitis, where morphological changes are irreversible 6