In patients with a segmentally or focally enlarged pancreas, AIP is important to differentiate from pancreatic ductal adenocarcinoma or mass-forming pancreatitis (Fig. 10.3). Wakabayashi et al. [9] reported that focal AIP is associated with clinical and radiologic features different from those of alcohol-induced mass-forming pancreatitis, but similar to those of pancreatic ductal adenocarcinoma. No dilation of the main pancreatic duct is seen in most cases of AIP, but in some patients dilation may make differential diagnosis difficult (Fig. 10.4). In AIP, US visualizes the main duct passing through the mass, which is called the “duct-penetrating sign.” This sign is useful in distinguishing AIP from pancreatic ductal adenocarcinoma [10].

Contrast-enhanced ultrasonography (CEUS) has been reported to be useful in distinguishing AIP from pancreatic ductal adenocarcinoma [6, 11–13]. In pancreatic ductal adenocarcinoma, CEUS displays enhancement only at the periphery of the mass, in associating with tumor vessels. In AIP, enhancement occurs in the entire mass, unassociated with tumor vessels. Morana et al. [6] reported that by CEUS, most inflammatory pancreatic masses showed a pattern of enhancement similar to that of normal pancreatic parenchyma (“isovascular”), while focal pancreatic tumors were hypovascular relative to surrounding parenchyma, and this criterion achieved sensitivity and accuracy in differentiating the 2 diseases of 98 and 95 %, respectively.

Numata et al. [11] correlated vascularity of pancreatic lesions on CEUS with pathologic findings (fibrosis and inflammation) in 6 patients with AIP. The AIP lesions exhibited mild (n = 1), moderate (n = 3), or marked (n = 2) enhancement in both early and delayed phases of the examination. A direct correlation was evident between enhancement (vascularity) and pathologic degree of inflammation, and an inverse correlation was noted between enhancement and degree of fibrosis associated with AIP. Vascularity of all 3 lesions with follow-up CEUS examinations had decreased after steroid therapy. Those authors concluded that CEUS appeared useful for evaluating both vascularity of AIP lesions and therapeutic efficacy of steroid therapy [11]. On the other hand, Miyoshi et al. [13] reported that CEUS findings in patients with AIP varied by stage of disease; areas with intense inflammation and immature fibrosis enhanced strongly, while areas with less intense inflammation and more established fibrosis enhanced weakly.

AIP frequently complicates IgG4-related sclerosing cholangitis (IgG4-SC), in such cases. US detects thickening of the wall of the common bile duct (Fig. 10.5), and frequency of bile duct wall thickening in AIP was reported to be about 60 % [8]. A thickened bile duct wall is characterized by layered or parenchymal hypoechoic wall thickening [14, 15]. In some cases, thickening of ducts wall beyond extrahepatic bile ducts extends to intrahepatic bile ducts. However, US cannot detect narrowing of the intrapancreatic bile duct, a feature included among the criteria for IgG4-SC [16]. Accordingly, strictures of hilar or intrahepatic bile ducts are assessed by other means in patients with AIP [16].

Duct wall thickening in AIP has been studied in detail with endoscopic ultrasonography (EUS) and intraductal ultrasonography (IDUS) [8, 17]. Although wall thickening in narrowed duct segments is not shown clearly by conventional US, EUS and IDUS depict circular, symmetric wall thickening with smooth outer and inner margins, associated with homogeneous internal echo in narrowed ducts [18, 19]. Sometimes, difficulty arises in correctly diagnosing IgG4-SC, when IDUS displays irregular wall thickening during the acute phase of disease. However, after steroid therapy, wall thickness has decreased conspicuously [20].