Dr. Takahashi, at the time this article was written, was a Clinical Fellow and Dr. Brown is an Associate Professor of Radiology at Mallinckrodt Institute of Radiology, Washington University Medical School, St. Louis, MO. Currently, Dr. Takahashi is an Instructor in the Department of Radiology, International Medical Center of Japan, Tokyo, Japan.

Magnetic resonance imaging (MRI) is commonly used as a problem-solving tool for imaging of the pancreas. With recent advances, such as breath-hold, dynamic contrast-enhanced imaging, and MR cholangiopancreatography (MRCP), MR is more often used as a primary imaging modality. MR has an accuracy similar to computed tomography (CT) in the detection, characterization, and staging of various pancreatic tumors.

MR protocol

At our institution, the standard MR protocol includes coronal half-Fourier acquisition single-shot turbo-spin echo (HASTE), axial in-phase and opposed-phase gradient-recalled echo (GRE), axial T2 turbo-spin echo with fat suppression, or turbo-inversion-recovery through the liver and pancreas. Two-dimensional (2D) spoiled GRE with fat suppression and three-dimensional (3D) spoiled GRE with fat suppression through the pancreas are also obtained. After administration of gadolinium contrast material at 2 mL/sec, we obtain breath-hold dynamic 3D spoiled GRE sequence in pancreatic, portal, and equilibrium phases through the pancreas. We start the pancreatic phase approximately 25 sec after the start of injection of contrast material. ¹ The portal phase begins approximately 60 sec, and the equilibrium phase approximately 100 sec, after the start of injection of the contrast. If islet cell tumor is suspected, the first phase should be obtained in the arterial phase, which occurs approximately 10 sec, before the pancreatic phase. We typically perform test bolus injection to determine the scan delay in this instance. A 2D spoiled GRE sequence with fat suppression can be used instead of a 3D spoiled GRE sequence for the dynamic study. With the contrast-enhanced dynamic GRE sequence, we typically cover only the pancreas to maximize the spatial resolution, and add one delayed spoiled GRE sequence to cover the entire liver. An MRCP sequence may be added as necessary before administration of contrast material. MR cholangiopancreatography is performed using half-Fourier rapid acquisition with relaxation enhancement (RARE). A thick-slab technique is used to provide a global view of the biliary tree and pancreatic duct. Thin-slice technique is useful for evaluation of detailed anatomy.

Normal MR appearance of the pancreas

On T1-weighted images without fat suppression, the pancreas is isointense or slightly hyperintense to the liver. With fat suppression, the normal pancreas is hyperintense to the liver or other solid organs in the abdomen. High signal intensity on T1-weighted images is due to the aqueous protein in the glandular tissue (Figure 1A). ² On T2-weighted images, the pancreas has similar signal intensity to the liver, and is hypointense compared with the spleen or kidneys. After administration of contrast material, the pancreas enhances maximally during the pancreatic phase in contrast to the liver, which shows minimal enhancement during this phase because of its predominant portal venous supply. During the portal and delayed phases, the pancreas becomes isointense to the liver (Figures 1B and C).

Congenital and hereditary abnormalities

Pancreas divisum is the most common congenital anomaly of the pancreas. Divisum occurs as a result of incomplete fusion of the ventral and dorsal pancreatic ducts. The dorsal pancreatic duct drains into the duodenum via the minor papilla, separately from the ventral pancreatic duct and common bile duct. An association between chronic pancreatitis and pancreas divisum has been suggested. ³ On MRCP, the aberrant drainage of the dorsal pancreatic duct can be visualized (Figure 2). Annular pancreas is a rare anomaly in which the pancreas encircles the second portion of the duodenum.

Primary hemochromatosis is characterized by abnormal iron deposition, which mainly affects the liver, heart, and pancreas. Involvement of the pancreas occurs in the late stage of the disease. ⁴ Because of the magnetic susceptibility effects of iron, the pancreas becomes hypointense on T2-weighted images and, to a lesser degree, on T1-weighted images.

Von Hippel-Lindau disease is a phakomatosis that primarily affects the central nervous system (hemangioblastoma) but may also affect other organs, including the kidneys, adrenal glands, and pancreas. ⁵ In the pancreas, simple cysts, islet cell tumors, and serous cystadenomas can be found.

Adenocarcinoma

Adenocarcinoma comprises more than 80% of pancreatic neoplasms. The majority (65%) of pancreatic carcinomas occur in the head of the pancreas. The prognosis is dismal if the tumor is not resected, and only 5% to 22% of patients are candidates for curative resection. Tumors in the head of the pancreas tend to present earlier than tumors in the body or tail due to obstructive symptoms. Tumors in the tail and body of the pancreas are rarely operable. The disease may spread through many pathways, such as: liver metastasis, lymph node metastasis, peritoneal carcinomatosis, and direct invasion of the surrounding structures, such as the celiac artery, superior mesenteric artery, portal vein, bile duct and duodenum. ⁶ Absolute contraindications for curative surgery include distant metastases, peritoneal carcinomatosis, and major arterial involvement. ⁶ Portal venous involvement is a relative contraindication. In some cases, a segment of the portal vein may be sacrificed and replaced with a venous graft.

The main roles of cross-sectional imaging techniques in the evaluation of patients with suspected pancreatic cancer are lesion detection, characterization, and staging. MRI has been shown to be similar or superior to helical CT for these purposes. ^7-9 Detection and staging of local spread of the disease is best assessed with dynamic GRE sequences. The tumor appears as an ill-defined area of hypointensity surrounded by normally enhancing pancreatic parenchyma in the early phase of enhancement (pancreatic phase) (Figure 3). Usually, the mass shows a thin rim of faint enhancement. In the equilibrium phase, the tumor tends to become isointense to the surrounding normal pancreas. The appearance of the common bile duct and main pancreatic duct should be evaluated carefully, particularly when a mass is not visualized in the pancreas. Dilation of both biliary and pancreatic ducts or abrupt termination of a dilated duct may suggest the presence of a mass (Figure 3). ¹⁰ MR cholangiopancreatography is the best approach for evaluating the ductal system. For evaluation of vascular invasion, the pancreatic and venous phases of the dynamic study are useful. Vascular invasion can be evaluated by the degree of tumor abutment of the vessels. Data from a CT study suggests that if the tumor abuts more than half the circumference of the vessel, it is likely that the vessel is invaded by the tumor (Figure 4). On the other hand, if the tumor abuts ¾ 25% of the vessel circumference, it is unlikely that the vessel is invaded by tumor. ¹¹ When 3D spoiled GRE sequence is used, multiplanar reconstruction (MPR) or other 3D rendering techniques are useful in assessing vascular invasion. Liver metastases are slightly hyperintense on T2-weighted images and hypointense on postcontrast GRE images. Lymph node metastases are best depicted on T2-weighted images and postcontrast GRE images. Noncontrast T1-weighted images are also useful for detecting lymph nodes and assessing vascular encasement by tumor.

Differentiating between carcinoma and focal pancreatitis can be extremely difficult. Chronic pancreatitis can cause mass effect and obstruct either the common bile duct or pancreatic duct. It can even form an inflammatory mass that is indistinguishable from carcinoma. In some cases, carcinoma can cause pancreatitis by obstructing the pancreatic duct, and the pancreatitis can obscure the mass.

Islet cell tumors

Islet cell tumors are neuroendocrine in origin, and they can be functional or nonfunctional. The tumors tend to be smaller when functional due to earlier presentation. Functional islet cell tumors include insulinoma, gastrinoma, VIPoma, glucagonoma, and somatostatinoma. Insulinoma is the most common of the islet cell tumors. Insulinoma most commonly occurs in the tail of the pancreas, and its average size at the time of presentation is <2 cm. ¹² Gastrinomas tend to be slightly larger than insulinomas and commonly occur in the head of the pancreas. Islet cell tumors appear hypointense on T1-weighted fat-suppressed images and hyperintense on T2-weighted images; they show early arterial enhancement on dynamic contrast-enhanced images (Figure 5). ^13-15 Thoeni et al ¹⁵ demonstrated that noncontrast T1-weighted fat-suppressed GRE images were the most sensitive for lesion detection because of the high signal intensity of the normal pancreas. Islet cell tumors are rarely cystic, ¹² and contrast enhancement may be present at the rim of the cystic mass. In patients with multiple endocrine neoplasms syndrome, islet cell tumors are often multiple.

Cystic neoplasms

Cystic neoplasms of the pancreas can be divided into two subgroups: serous cystadenoma and mucinous cystic neoplasms. Serous cystadenoma is a benign neoplasm, often referred to as microcystic cystadenoma. It typically consists of clusters of small cysts <2 cm in diameter, although a macrocystic variant has been reported. ¹⁶ On T2-weighted images, serous cystadenoma is a lobulated mass with very high signal intensity. ¹⁷ The walls of each septa may be imperceptible (Figure 6). Stellate calcification may be visible as a signal void, although MR is less sensitive to calcium than CT. After contrast administration, diffuse faint contrast enhancement can be seen because of wall enhancement within small cysts. There is considerable overlap in the imaging appearance of serous and mucinous cystic neoplasm; therefore, unless the lesion has a completely typical appearance for serous adenoma, it should be surgically removed.

Mucinous cystic neoplasm is often referred to as macrocystic cystadenoma or cystadenocarcinoma. Such lesions should be considered either premalignant or frankly malignant. These lesions commonly occur in the body or tail of the pancreas in women between the ages of 40 to 70 years. Mucinous cystic tumors typically consist of clusters of cysts, with each cyst >2 cm in diameter. On T2-weighted images, mucinous cystic neoplasms have high signal intensity and internal septations. ¹⁷ There is no contrast enhancement within the cystic component of the mass (Figure 7). On T1-weighted images, the cyst may be hyperintense due to mucin content. The walls of the cystic mass may be variable in thickness and there may be nodular enhancing areas. Thick walls and the presence of mural nodules are suggestive of malignancy, although absence of these findings does not exclude malignancy. Since it is often very difficult to differentiate between mucinous adenoma and adenocarcinoma by imaging, mucinous tumors usually require resection.

Intraductal papillary mucinous tumor (IPMT) is a subtype of the mucinous cystic neoplasms. The tumor arises from the epithelium of the main pancreatic duct or its branches, and grows into the pancreatic duct in a papillomatous pattern. The tumor produces copious amounts of mucin; thus the tumor is often referred to as mucin hypersecreting tumor of the pancreas. The hallmark of the main-duct type IPMT is dilatation of the main pancreatic duct, which may be indistinguishable from chronic pancreatitis (Figure 8). On MRCP, pancreatic duct dilatation is well depicted, and branch-duct type IPMT appears as a cluster of small cysts with or without septation. ¹⁸ Communication between the cysts and main pancreatic duct can often be visualized, which may help in differentiation from a pseudocyst. Intraductal papillary mucinous tumor has variable malignant potential. Findings that favor malignant IPMT include the presence of filling defects or papillary projections within the dilated pancreatic duct or cysts and dilatation of the main pancreatic duct (>15 mm for main-duct type and any dilation for branch-duct type). ¹⁹

Solid and papillary epithelial neoplasms

Solid and papillary epithelial neoplasm of the pancreas is a rare tumor that occurs mainly in women under 40 years of age. The mass is usually large (>10 cm) and commonly exhibits cystic and hemorrhagic degeneration from necrosis with a thick rim of soft tissue. On T1-weighted images, the hemorrhagic component appears hyperintense, ²⁰ and the soft-tissue rim enhances after administration of gadolinium contrast (Figure 9).

Other cystic masses

Pancreatic pseudocyst is the most common cystic lesion in the pancreas (Figure 10). It is a complication of pancreatitis, and is often seen in association with changes of pancreatitis. Pancreatic abscess is a rare complication of pancreatitis, and can occur secondary to superinfection of pancreatic necrosis or pseudocyst (Figure 11). True epithelial cysts are rare in the pancreas but may occur in patients with polycystic kidney disease or von Hippel-Lindau disease (Figure 12). As noted, islet cell tumors are rarely cystic. Lymphoepithelial cyst (lymphangioma) is a rare cystic tumor that is often indistinguishable from mucinous cystic neoplasms. ²¹

Metastases

Common primary malignancies that can metastasize to the pancreas include carcinoma of the lung, breast, and kidney, as well as melanoma. Metastases may involve the pancreas via local invasion from the peripancreatic lymph nodes. Typically, metastases are hypointense on T1-weighted images, and hyperintense on T2-weighted images compared with the surrounding pancreas. Metastases from melanoma may be hyperintense on T1-weighted images due to the paramagnetic properties of melanin. ¹³ After administration of gadolinium contrast, the enhancement pattern of the metastatic lesion often simulates that of the primary tumor. Metastasis from renal cell carcinoma is often hypervascular.

Pancreatitis

Acute pancreatitis

The diagnosis of acute pancreatitis is usually made clinically, and MR is mainly performed to exclude complications, or, more importantly, to find the cause of pancreatitis, such as choledocholithiasis or pancreas divisum. MR cholangiopancreatography is an essential sequence to look for choledocholithiasis as a cause of acute pancreatitis. ²² On MR, the pancreas may appear normal or show subtle enlargement. In more severe cases, the signal intensity of the pancreatic parenchyma becomes heterogeneous on T1-weighted fat-suppressed images and postcontrast GRE images in the pancreatic phase secondary to edema. Pancreatic necrosis, an important prognostic indicator, appears as a focal perfusion defect on postcontrast images. ²³ In the case of hemorrhagic pancreatitis, fluid collections with a hemorrhagic component are present in or around the pancreas. The fluid collections are typically hyperintense on T1-weighted images due to methemoglobin.

Chronic pancreatitis

Morphological changes of chronic pancreatitis, which occur in the late stage of the disease, include atrophy and pancreatic duct dilatation. Pancreatic calcification is not usually visible on MR unless it is severe. The signal changes if the pancreatic parenchyma precedes the morphological changes in the early stage of the disease, which include decreased signal on both T1- and T2-weighted images and heterogeneous enhancement secondary to fibrosis. ²⁴ As already noted, the chronic pancreatitis can be focal and may mimic pancreatic carcinoma (Figure 13). MR pancreatography performed before and after secretin administration has been investigated to assess exocrine function reserve in chronic pancreatitis. ²⁵

Mangafodipir trisodium

Mangafodipir trisodium (MnDPDP) (Teslascan, Amersham Health, Princeton, NJ) is a T1-shortening contrast agent (hyperintense on T1-weighted images) due to the paramagnetic nature of manganese. It was originally developed as a liver-specific contrast agent, but the agent is also taken up by normal pancreas. The signal intensity of normal pancreatic tissue increases while the signal intensity of pancreatic lesions remains the same, thereby increasing lesion conspicuity on T1-weighted images. ²⁶ Although the contrast-to-noise ratio is lower than on the images acquired with gadolinium-based contrast agents, the temporal window for imaging is longer since the washout of the MnDPDP takes more than 1 hour. The clinical role of MnDPDP in the evaluation of pancreatic disease has yet to be established.

Conclusion

Pancreatic MRI is primarily used as a problem-solving tool or when CT is contraindicated. Recently, MR has been used more often as a primary diagnostic modality. MRI has similar accuracy to CT in detecting and staging of pancreatic tumors, and MR may have an added role in characterizing lesions. AR