**5. Autoimmune pancreatitis**

Autoimmune pancreatitis (AIP) is an uncommon form of pancreatitis characterized by frequent the presentation of focal or diffuse pancreatic enlargement with (when the lesion affects the pancreatic head) or without obstructive jaundice, caused by a histological lymphoplasmacytic infiltrate and fibrosis and characterized by a dramatic response to steroids [32]. Two forms of AIP are classified based on clinical and histopathological findings. AIP type 1 is characterized by lymphoplasmacytic sclerosing pancreatitis without granulocyte epithelial lesions, with the presence of dense infiltrate of IgG4 positive plasma cells, being an expression of an IgG4-related systemic disease often characterized by extrapancreatic lesions (i.e., sclerosing cholangitis, sclerosing sialadenitis, and retroperitoneal fibrosis). AIP type 2, on the other hand, refers to idiopathic duct-centric pancreatitis with granulocyte epithelial lesions; this condition does not appear to be associated with IgG4-related systemic disease but it can be associated with an inflammatory bowel disease. In this latter condition, pancreatic enlargement may not be visible but stenosis of the Wirsung duct is characteristic.

The role of radiology in AIP is fundamental, as it can characterize it and distinguish it from PDAC in the case of focal AIP, allowing correct management of this disease.

MRI with MRCP allows both to evaluate the pancreatic parenchyma, its enhancement, and the morphology of the ductal system [33, 34]. Furthermore, DWI imaging, evaluating the microscopic movement of water molecules, can show a restricted diffusion due to the presence of cellular infiltrate that limits the water movement. The imaging findings must be understood considering that histopathology of AIP includes a periductal infiltrate which leads to an increase in the size of the pancreatic gland and at the same time to a compression of the ductal system; another important aspect that characterizes AIP is the presence of obliterating vasculitis [32, 35, 36]. This is important because these two factors, hypovascularization and ductal stenosis, can radiologically mimic pancreatic cancer. Therefore, AIP exhibits imaging features that may be typical or atypical.

Typical AIP is characterized by diffuse ("sausage-like") enlargement of the gland with a loss of physiological lobulation due to an increase in tissue pressure with

## *Imaging of Pancreatitis DOI: http://dx.doi.org/10.5772/intechopen.106764*

delayed enhancement, sometimes associated with rim-like enhancement. In AIP at MRCP the main pancreatic duct presents a long reduction of the caliber (more than 1/3 of the length of the main pancreatic duct) or multiple strictures without significant upstream dilatation (**Figure 11**), differently from pancreatic cancer where the stenosis is focal with marked upstream dilatation.

However, some AIP presents with an atypical appearance, showing low-density focal mass at CT, pancreatic duct upstream dilatation, or distal atrophy of the parenchyma. These atypical imaging findings in patients with obstructive jaundice are highly suggestive of pancreatic cancer and should be managed as pancreatic cancer unless there is strong evidence for AIP, and a precise workup for cancer is negative [32]. However, the presence of segmental or focal enlargement of the pancreas with delayed enhancement, highly restricted diffusion and focal narrowing of the main pancreatic duct without marked upstream dilatation (<5 mm), are imaging signs that may aid in the differential diagnosis between atypical AIP and pancreatic cancer

### **Figure 11.**

*M, 55 yo. Autoimmune pancreatitis, diffuse form. A diffuse enlargement of the pancreas is appreciable (a), showing restricted diffusion with high signal intensity on DWI (b), hypovascular in the arterial phase (c) but showing late homogeneous enhancement (d). At MRCP (e), the Wirsung duct is not appreciable due to the compression from the enlarged pancreas but returns to normal after steroid therapy (f).*

### **Figure 12.**

*M, 69 yo. Jaundice. Autoimmune pancreatitis, focal form. A focal enlargement of the head of the pancreas is appreciable (a), showing restricted diffusion with high signal intensity on DWI (b), hypovascular in the arterial phase (c) but showing late homogeneous enhancement (d). At MRCP (e), marked stenosis both of the Wirsung duct and choledochus is appreciable, simulating a "double duct sign", typical of PDAC. After steroid therapy (f) both Wirsung duct and choledochus return to normal.*

(**Figure 12**). If the stenosis is focal and single, the possibility of pancreatic cancer is high, but if the stenosis is two or more, it could be a case of atypical AIP [37, 38].
