Imaging findings are often the first and most important clues for the diagnosis of AIP. In fact, their presence is mandatory in the Japanese and Asian diagnostic criteria for AIP.[45,46] We will now review the common imaging characteristics of AIP.
Transabdominal ultrasound is one of the most common tests performed for obstructive jaundice and pain; however, pancreatic imaging on transabdominal ultrasound is often suboptimal owing to bowel gas and large-body habitus. Visualized pancreas may show diffuse hypoechogenicity and thickened bile duct, features of AIP that are better viewed on endoscopic ultrasound (EUS). However, it can be of help in diagnosing gallstone disease. In the future, there may be a role for ultrasound elastography and other augmented ultrasound techniques.
Cross-sectional imaging is an important investigation in the differential diagnosis of obstructive jaundice and pancreatic mass. Widespread availability of CT scans and expertise available to interpret the images makes CT the first choice in cross-sectional imaging in suspected pancreatic cancer. A large, bulky, 'sausage-shaped' pancreas with delayed and rim enhancement is pathognomonic of AIP.[19,48,49,50] In the arterial phase, the pancreas is less dense than the spleen but slowly enhances in the delayed phase. Even in the absence of the rim-like enhancement, the sausage-shaped enlargement with delayed enhancement in and of itself is highly suggestive of AIP.[48,49] This is a common feature in AIP.
When the involvement is focal rather than diffuse the CT appearance can be that of a focal enlargement and much less commonly a low density mass. In both the diffuse and focal forms there is a characteristic lack of dilatation of pancreatic duct and lack of atrophy of the pancreas. In addition to the pancreatic findings, CT scans can also offer extra pancreatic clues to the diagnosis of AIP: focal dilatation of intrahepatic bile ducts, bulky intra-abdominal adenopathy, renal parenchymal involvement and retroperitoneal fibrosis.
MRI/Magnetic Resonance Cholangiopancreatography
MRI may be comparable to a CT scan to visualize the pancreatic parenchyma, but greater cost and less widespread availability limit its usage. Its advantages are lack of exposure to radiation (especially useful in relapsing diseases requiring follow-up scans) and its ability to visualize the bile ducts on magnetic resonance cholangiopancreatography (MRCP). An enlarged pancreas with decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted images (compared with signals of the liver) can be seen. The hypodense capsule-like rim may also be seen as well.[49,51] An attenuated pancreatic duct, characteristic of AIP, is often not visualized on MRCP.
Endoscopic Retrograde Pancreatography
The most reliable finding on endoscopic retrograde pancreatography (ERP) suggestive of AIP is a long (more than a third of the pancreatic duct) narrowing with no upstream dilatation. This most likely represents the diffuse type of AIP. The presence of multiple noncontiguous narrowing is also suggestive of AIP. The focal type of AIP presents a diagnostic challenge on ERP, as it is very similar in appearance to pancreatic cancer. The diversity in the ERCP findings is probably owing to the extent of the gland involved and the amount of desmoplastic reaction generated. In addition to the pancreatic duct involvement, the bile duct can also be involved conjunctively or separately. Involvement of the intrapancreatic bile duct mimics chronic pancreatitis (CP) or pancreatic cancer, and proximal and intrahepatic bile duct involvement mimics cholangiocarcinoma and PSC.[52,53,54,55,56] The histology of the ampulla and surrounding tissue has been shown to aid the diagnosis of AIP, especially when IgG4 stains are used, but this feature has not been incorporated in the diagnostic algorithms.[57,58] There can be dramatic improvement in strictures after treatment with corticosteroids.
Endoscopic ultrasound is the best adjunct to a CT scan in diagnosing AIP. There are some classic appearances that have been described on EUS, such as a diffusely enlarged hypoechoic gland. The greatest ability of EUS is its ability for obtaining tissue. EUS can be used to obtain FNA to exclude pancreatic cancer, and core biopsies can be obtained in specialized centers for a more definitive histologic diagnosis of the lesion. However, the negative predictive value of EUS FNA for pancreatic cancer is approximately 75% and, in the best hands, core biopsies are diagnostic of AIP in approximately 75% of patients.[20,59] However, there are technical limitations to tissue sampling, especially when the head of the pancreas is involved. Core biopsy of the pancreatic head is technically difficult as the angulation imposed on the endoscope does not allow easy passage of the needle and its sheath.[60,61]
In the future, intraductal ultrasound (IDUS) and EUS elastography may be valuable tools in the diagnostic armamentarium.[62,63,64] In addition, there is an interesting study researching PET in AIP. These newer imaging techniques are in the very preliminary stage, and further studies are needed before they can be incorporated in the diagnostic algorithms.
Expert Rev Gastroenterol Hepatol. 2009;3(2):197-204. © 2009 Expert Reviews, Ltd.
Cite this: Autoimmune Pancreatitis: An Update - Medscape - Apr 01, 2009.