Pancreas: Imaging Approach and Differential Diagnosis

Embryology and Normal Variants

The body-tail segment of the pancreas develops from the embryologic dorsal pancreatic bud, while the head-uncinate segments develop from the ventral bud, which also gives rise to the liver and biliary tree.

During normal development, the ventral bud migrates clockwise around the fetal duodenum and eventually merges with the dorsal bud to form the pancreas with the branching pancreatic and biliary ducts in communication.

Variations in this process are relatively common, including many variations of the pancreatic duct branching pattern. Among the most common is pancreas divisum , in which there is no communication between the accessory duct of Santorini and the main duct of Wirsung that drains the pancreatic head.

Errors of rotation and fusion may also result in a completely circumferential or annular pancreas , which may be associated with pancreatitis as well as obstruction of the duodenal lumen.

As a result of their separate embryologic origins, the ventral and dorsal pancreatic segments may exhibit varying morphology, ranging from variations in the degree of fatty lobulation to complete absence of the dorsal pancreas. Both may simulate serious pathology unless one is familiar with these normal variants.

Pancreatic size and the degree of fatty lobulation vary substantially within the population based on patient age, body habitus, and other factors. In patients over the age of 70, the parenchyma atrophies, often develops small foci of calcification, and the pancreatic duct dilates slightly. These senescent changes should not be interpreted as evidence of chronic pancreatitis without corroborating clinical evidence.

Imaging Protocols

US may be the 1st study performed in the evaluation of abdominal pain. The pancreas, however, is often incompletely evaluated secondary to overlying bowel gas. If a cystic mass is identified, color Doppler should always be performed to rule out a vascular lesion. For most lesions, the pancreas is far better evaluated with CT and MR using multiphasic and multiplanar imaging .

Most ductal carcinomas are best detected as hypoenhancing masses relative to the normal pancreas, while endocrine (islet cell) tumors are hypervascular. Both types of tumors may be evident only on the arterial or pancreatic parenchymal phase of imaging (~ 35- to 45-second delay). Hepatic metastases from endocrine tumors may also be evident only on arterial-phase images. Portal venous (hepatic parenchymal)-phase images remain necessary in order to evaluate hypovascular metastases, venous involvement, and other anatomic features.

CT protocol : We often obtain a series of noncontrast scans through the liver and pancreas to help in recognition of small foci of calcification and degree of tumor enhancement and to help in localization for subsequent series. Since some hypervascular tumors (e.g., gastrinoma) may arise in the wall of the duodenum, and because multiplanar CT angiography is so often a useful adjunct, it is best to give only water or a neutral oral contrast agent rather than barium or iodinated contrast that may obscure intramural lesions or make CT angiography difficult.

Multiphasic scanning : Arterial (pancreatic parenchymal)-phase images should be obtained through the liver and pancreas as contiguous 0.65- to 1.25-mm images, reconstructed for viewing as 2.5- to 5.0-mm thick sections. The thinner source images are required for optimal multiplanar reformations. Portal venous (hepatic parenchymal)-phase images should be obtained through the liver and pancreas as well, using the same parameters.

Multiplanar reformations are very useful in recognizing and characterizing pancreatic disease. Curved planar reformations along the pancreatic duct offer the most compelling evidence of abrupt narrowing by a tumor mass or chronic pancreatitis. Variations of coronal and sagittal planes frequently make peripancreatic nodal involvement and local invasion more evident and may even be essential in recognizing a lesion as being intra- or peripancreatic in origin (e.g., an accessory spleen vs. an islet cell tumor in the tail of the pancreas).

MR protocols : There are many vendor-specific pancreatic protocols, but the basic examination should include gradient-echo T1-weighted imaging in and out of phase, T2-weighted images, and dynamic gadolinium-enhanced gradient-echo images with fat suppression. An MRCP should be performed in cases of suspected pancreatic pathology to evaluate both the pancreatic and biliary ducts.

A pproach to the Abnormal Pancreas

Unlike other abdominal organs, the pancreas has only a thin capsule that is easily breached by inflammatory and neoplastic processes. Processes that originate within the pancreas can easily spread to adjacent structures, including other viscera within the anterior pararenal space, such as the duodenum and vertical colon segments. Pancreatitis, for instance, often affects the duodenal lumen and may cause inflammation of the descending colon as well. Pancreatic malignancies commonly invade the adjacent viscera, and even more commonly adjacent vessels and nerves, largely accounting for their poor prognosis. Invasion into the lumen of the splenic vein, which runs within the pancreas, leads to liver metastases; while occlusion of the splenic vein leads to characteristic perigastric varices in the absence of esophageal varices or cirrhosis. This may constitute the most obvious sign on imaging of a small primary tumor.

Obstruction or dilation of the pancreatic &/or common bile duct is another indirect but useful sign of pancreatic disease. Pancreatic ductal dilation is most frequently due to ductal carcinoma or chronic pancreatitis but may also result from intraductal papillary mucinous neoplasm (IPMN), neuroendocrine tumor, or even metastases to the pancreas.

Obstruction of the intrapancreatic bile duct may result from the same inflammatory and neoplastic processes of the pancreas. Obstruction of only the distal common bile duct with a normal pancreatic duct is more likely the result of primary inflammatory, infectious, neoplastic, or calculous disease originating in the biliary system.

Pancreatic ductal carcinomas are hypovascular tumors and often present on imaging as hypodense (hypointense) masses with abrupt obstruction of the ducts. Metastases to the pancreas from primary tumors, such as lung, breast, or melanoma, may simulate a primary ductal cancer.

Neuroendocrine tumors are more often hypervascular and may be simulated by metastases from hypervascular primary tumors, such as renal cell carcinoma.

Cystic pancreatic masses are discovered frequently on CT, MR, or US, making a pragmatic approach to their evaluation mandatory. Clinical history is necessary, as is consideration of the age, gender, and presence of laboratory evidence of pancreatitis. For example, mucinous cystic neoplasms occur almost exclusively in young or middle-aged women, while solid and papillary epithelial neoplasms occur in girls and young women. Pseudocysts are often indistinguishable from cystic tumors by imaging alone but usually occur in patients with known pancreatitis and evolve in size much more rapidly than tumors. Certain cystic tumors may have a characteristic appearance, allowing confident diagnosis and management; others will require further evaluation with endoscopic US emerging as the single most accurate means of guiding diagnosis and management in more complex cases.

Aneurysmal dilation of a peripancreatic artery (or varix) should be considered in the differential diagnosis of any cystic pancreatic mass. The presence of flowing blood must be determined by rapid IV administration of contrast medium for CT or MR or by color Doppler sonography.

Pancreatitis and complications of pancreatitis are commonly encountered in abdominal imaging. Because the pancreas is separated from the lesser sac by only the posterior parietal peritoneum, acute pancreatitis often results in exudation of fluid into the lesser sac, which should not be mistaken for a pseudocyst. These fluid collections usually resolve quickly, while pseudocysts take longer to resolve and have, by definition, a fibroinflammatory wall.

The tail of the pancreas is the distal few centimeters of the gland that lies within the splenorenal ligament and is actually intraperitoneal. Acute inflammation of the pancreatic tail may result in an intrasplenic pseudocyst and pancreatic ascites. Tumors arising from the pancreatic tail can easily invade the spleen without crossing an anatomic boundary. An important potential pitfall in this area is the accessory spleen, which may be located within the tail and mimic a hypervascular pancreatic mass.