A 59-year-old woman with typical imaging features of autoimmune pancreatitis in the pancreatic (a) and portal venous phase images (b) of CT showing mild diffuse enlargement of the pancreas with loss of the lobular architecture (“sausage-shaped pancreas”), hypoattenuation in the pancreatic phase and hyperattenuation on the portal venous phase (“delayed enhancement”). Note that the pancreas enhances less than the normal pancreas in the pancreatic phase (a) and then exhibits delayed enhancement in the portal venous phase (b). Also note the paucity of peripancreatic stranding
A 56-year-old man with typical imaging features of autoimmune pancreatitis. (a) Axial portal venous phase CT image shows mild diffuse enlargement of the pancreas with hypoattenuating capsule-like rim surrounding the pancreas. (b) Axial T2-weighted MR image shows diffuse enlargement of the pancreas appearing as moderate hyperintensity with hypointense capsule-like rim around the pancreas. (c) MRCP image shows diffuse irregular luminal narrowing of the main pancreatic duct
On CT or MRI, autoimmune pancreatitis enhances less than normal pancreas in the pancreatic phase (or arterial phase) and then demonstrates delayed enhancement in the portal venous and equilibrium phase in over 90 % of cases [2, 7–9]. This delayed enhancement pattern of autoimmune pancreatitis is one of the most important imaging features and useful in differentiating from other diseases, which will be discussed afterward in this chapter. On MRI, the pancreas appears moderately hyperintense on T2-weighted images and hypointense on T1-weighted images in autoimmune pancreatitis patients (Fig. 11.2).
Hypoattenuating or hypointense capsule-like rim surrounding the pancreas is a supplementary finding of autoimmune pancreatitis seen in 12–40 % of cases on CT and MRI [2, 6, 10] (Fig. 11.2). The rim reflects intense inflammatory infiltration and fibrosis of the peripancreatic tissue [8, 10]. On MRI, the rim appears hypointense on T1- and T2-weighted images and delayed enhancement. Peripancreatic fat stranding, which is a common feature in other causes of pancreatitis, is usually minimal in autoimmune pancreatitis  (Fig. 11.2). Regional lymphadenopathy and involution of the pancreatic tail can occur in patients with autoimmune pancreatitis [1, 5].
Another imaging hallmark of autoimmune pancreatitis is an irregular luminal narrowing of the main pancreatic duct, of which MRCP is the modality of choice [11, 12] (Fig. 11.2). Typically, there is long (>1/3 of the main pancreatic duct) or multiple strictures in the main pancreatic duct without marked upstream dilatation. And segmental or focal narrowing of the main pancreatic duct without marked upstream dilatation in atypical cases . However, ERCP is necessary if there is any symptomatic biliary obstruction [11, 12].
Of the two known forms of autoimmune pancreatitis, type 1 (lymphoplasmacytic sclerosing pancreatitis) is associated with IgG4-related disease and tends to develop in middle-aged and elderly men (mean age, 59–68 years) with a male-to-female ratio of 4–7.5:1 [13, 14]. The prevalence rate is 2–11 % among patients with chronic pancreatitis . On the other hand, autoimmune pancreatitis type 2 is seen in a younger population in comparison with patients with autoimmune pancreatitis type 1, which lacks elevated IgG4 levels. However, there was no imaging feature of the pancreas reported to differentiate the two types of autoimmune pancreatitis. And the two types may be definitively distinguished only by histopathologic assessment .
Atypical Findings and Differential Diagnosis
Autoimmune pancreatitis sometimes exhibits atypical imaging features, which makes the diagnosis difficult. Therefore, the diagnosis of autoimmune pancreatitis requires a combination of clinical, laboratory, imaging, and histologic findings.
The most important atypical imaging finding in autoimmune pancreatitis is focal pancreatic mass-like lesion in 30–40 % of cases, resulting from localized involvement of autoimmune pancreatitis seen commonly in the pancreatic head  (Fig. 11.3). The focal form of autoimmune pancreatitis may simulate pancreatic ductal adenocarcinoma, and there are ancillary imaging features helpful in distinguishing the two diseases; prominent lymphadenopathy, vascular occlusion, abrupt narrowing of the pancreatic duct, and marked atrophy of the pancreas distal to the mass increase the possibility of pancreatic carcinoma rather than autoimmune pancreatitis. The pancreatic duct penetrating through the mass-like lesion, known as “duct-penetrating sign,” is considered to be helpful for differentiating focal form of autoimmune pancreatitis from pancreatic cancer even though the finding has low specificity as up to 50 %  (Figs. 11.3 and 11.4). Although both autoimmune pancreatitis and pancreatic carcinoma show restricted diffusion (Fig. 11.3), diffusion-weighted MRI has been reported to be helpful in differentiation between autoimmune pancreatitis and pancreatic carcinoma [17, 18]. In a series of Kamisawa et al., the apparent diffusion coefficient (ADC) values were lower in patients with autoimmune pancreatitis than in those with pancreatic carcinoma. A possible ADC cutoff value (1.075 × 10–3 mm2/s) can be used in distinguishing between the two entities . Histological confirmation, however, remains for a diagnosis for many cases of focal autoimmune pancreatitis for differentiation from pancreatic carcinoma. Moreover, although rare in prevalence, pancreatic carcinoma may occur concurrently or during follow-up for autoimmune pancreatitis, even after histological confirmation of the diagnosis [19, 20].
A 56-year-old man with focal autoimmune pancreatitis. A round mass in the pancreas head appearing as moderate hyperintense lesion on (a) axial and (b) coronal T2-weighted MR images. (c) On MRCP image, the distal common bile duct shows segmental narrowing at level of the mass-like lesion in the pancreas head, but patency was preserved (“duct-penetrating sign”). (d) Diffusion-weighted image obtained at b value of 400 s/mm2 and (e) apparent diffusion coefficient map show restricted diffusion of the mass-like lesion in the pancreas head. After steroid treatment, the mass-like lesion in the pancreas head and distal CBD narrowing disappeared on (f) axial CT and (g) ERCP images
A 64-year-old man with pancreatic ductal adenocarcinoma. (a) On axial contrast-enhanced CT image, mild dilatation of the main pancreatic duct and cutoff at body portion was observed without any visible mass lesion. Further evaluation was performed under the suspicion of isoattenuating pancreatic cancer. (b) On axial precontrast T1-weighted MR image, it shows hypointense mass at the portion of duct cutoff. (c) MRCP image also shows abrupt termination of the main pancreatic duct (arrow) at the level of mass. Duct-penetrating sign is negative here, which suggests pancreatic carcinoma
Diffuse autoimmune pancreatitis must be differentiated from other causes of pancreatitis or diffuse pancreatic infiltration. Features that favor autoimmune pancreatitis over acute pancreatitis include the presence of a peripancreatic halo or capsule-like rim, minimal or no peripancreatic fat stranding, and the absence of peripancreatic necrosis [5, 21]. The presence of causative factors, such as gallstones, in addition to imaging features of acute pancreatitis, such as peripancreatic fat stranding and fluid collections, and necrosis of the pancreas or peripancreatic tissue reduce the likelihood of autoimmune pancreatitis (Fig. 11.5) Pseudocyst is a rare finding of autoimmune pancreatitis [5, 22], but often combined in patients with alcohol-induced pancreatitis .
A 42-year-old man with acute alcoholic pancreatitis. (a) The arterial phase and (b–c) portal venous phase images of axial contrast-enhanced CT show extensive peripancreatic fat stranding (arrows) and peripancreatic fluid collection in addition to diffuse enlargement of the pancreas. These imaging features point toward acute pancreatitis rather than autoimmune pancreatitis
Another differential diagnosis of autoimmune pancreatitis is lymphoma. Multiple and widespread lymphadenopathy and diffuse pancreatic infiltration would suggest lymphoma. Also, the delayed enhancement pattern of autoimmune pancreatitis is useful in differentiating from lymphoma and ductal adenocarcinoma, which exhibit lower degree of contrast enhancement compared to that of autoimmune pancreatitis in both of the pancreatic and portal venous phases [8, 9, 24].
Imaging Features After Treatment
Recent evidence suggests that most patients have been found to have significant symptomatic improvement after treatment with corticosteroids, and this is usually accompanied by radiological resolution [3, 25, 26] (Fig. 11.3). However, the resolution of radiological features is dependent upon the degree of fibrosis, and inflammatory change is more likely to resolve quickly. Therefore, a long-standing fibrotic disease may not change appearance with treatment, and extrapancreatic manifestations may resolve at a different rate from the pancreatic findings .