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 [1] (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 % [16] (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 [18]. 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].

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 [23].

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].

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 [27].