Imaging techniques for the pancreas (Proceedings)
Pancreatitis is a common consideration in dogs and in an increasing number of cats presented for vomiting, anorexia, lethargy, or abdominal pain. The disease however, is difficult to diagnose definitively, especially in cats. Clinicopathologic data, including amylase and lipase values are used routinely when canine pancreatitis is suspected. However, they may be normal, or elevated from other disease processes. They are not useful in cats. Newer tests, including canine and feline PLI are being used with increasing frequency, and are now commercially available. They appear to be fairly sensitive for pancreatits, but results are not available for several days. Imaging techniques have become an essential part of the workup in patients suspected of having pancreatitis.
Radiology of the pancreas
The normal canine pancreas is not seen on abdominal radiographs. However, in cats, the left pancreatic limb can often be seen as a thin linear soft tissue opacity extending to the left, between the gastric fundus, cranial pole of the left kidney, and spleen.
1. Loss of abdominal detail, primarily in the right cranial abdomen, due to focal peritonitis
2. Mass effect in the right cranial abdomen
3. Displacement of the pylorus cranially, or to the left
4. Ventral or right sided displacement of the descending duodenum
5. Caudal displacement of the transverse colon
6. Bowel loops adjacent to the pancreas (usually duodenum) may be gas-filled (ileus), or corrugated/spastic in appearance
Radiographs are frequently normal in dogs and cats with pancreatitis. In a previous study, only 24% (10/70 cases) of dogs with acute pancreatitis had radiographic changes consistent with this disease. Computed tomography is a sensitive means of making a diagnosis of pancreatitis or pancreatic neoplasia in people, but has limited availability, is expensive, and requires general anesthesia in veterinary medicine. Ultrasound, therefore, has become the imaging modality of choice in the diagnosis of pancreatic disease. Ultrasound examination of the pancreas can be difficult, however. The canine pancreas, unlike the liver or spleen, is often not seen as a discrete organ, and therefore the pancreatic region and its anatomic landmarks must be examined. The pancreas is surrounded by gas filled structures, including the duodenum, stomach, and colon, which may obscure visualization. Finally, abdominal pain in patients with pancreatitis may prevent the firm pressure on the transducer needed to visualize the pancreatic area.
Ultrasound of the pancreas
The pancreatic body joins the two lobes and is located immediately ventral to the portal vein and cranioventral to the right kidney. The body lies caudal to the pyloric region.
The left pancreatic lobe may be more difficult to visualize due to surrounding gas-filled structures (stomach and colon). It may be located in a triangular region cranial to the left kidney, caudal to the stomach, and medial to the spleen in the left cranial abdomen. It lies caudo-dorsal to the stomach and craniodorsal to the transverse colon.
The normal canine pancreas may on occasion be visualized, especially when using high frequency (7-10 MHz) transducers. It appears slightly more echogenic than the liver, and slightly less than the spleen.
Unlike the pancreas in the dog, the feline pancreas is typically visible as a discrete organ on ultrasound examination. It appears as a hypoechoic (similar to liver echogenicity) linear, well marginated organ immediately caudal to the stomach. The pancreatic duct extends through the center of the pancreas, visualized as a well demarcated anechoic tubular structure. In fact, visualization of the pancreatic duct is sometimes key to locating the pancreas. The pancreatico-duodenal vein, often prominent in the dog, is usually not seen
The right limb of the feline pancreas is small and less routinely visualized, but is located dorsomedial to the descending duodenum. The body and left limb of the feline pancreas are more consistently imaged, especially if using higher frequency transducers (7MHz or higher). The pancreatic body is identified directly caudal to the pylorus-duodenal angle, medial to the proximal duodenum, and ventral to the portal vein. The left limb can be identified caudal to the stomach, cranial to the left kidney, and medial to the spleen. In an imaging plane parallel to and adjacent to the left costal arch, the longitudinal image of the left limb may be followed laterally to the spleen.
Pancreatic pseudocysts are focal collections of pancreatic enzymes, blood, and products of tissue digestion within the pancreas. Eventually, a thick fibrous or granulation tissue capsule forms around the fluid collection. Pseudocysts are associated with acute pancreatitis, but require several weeks to develop. If small (less than 4 cm), they may resolve on their own. Larger or persistent (more than 6 weeks) pseudocysts require surgical drainage or removal. On ultrasound examination, they appear generally hypoechoic or anechoic, may have acoustic enhancement, and usually contain echogenic debris. Often, they are surrounded by a thick wall. It is difficult to differentiate a pancreatic pseudocyst from a pancreatic abscess on ultrasound exam alone. A needle aspirate may be required for a definitive diagnosis. One report of percutaneous ultrasound guided drainage of a pancreatic pseudocyst resulted in gradual resolution of the cyst. Complications of a persistent untreated pseudocyst include secondary infections, rupture, and hemorrhage.
Abscesses are collections of purulent material and necrotic tissue within the pancreas or extending into adjacent tissues. Pancreatic abscesses develop from secondary infection of necrotic pancreatic tissue, and are a serious complication of acute pancreatitis. They are often fatal unless there is surgical intervention. It is difficult to differentiate between pancreatic abscesses, pseudocysts, and even neoplastic disease. Abscesses may appear as ill defined mass lesions within the pancreas, containing both hypoechoic and hyperechoic areas. A thick or poorly defined wall may surround the abscess. These changes are similar to pseudocysts and tumors, so a needle aspirate or surgical biopsy is needed for a definitive diagnosis.
Pancreatic nodular hyperplasia
Nodular hyperplasia of the pancreas appears to be common in older cats and dogs, and is likely not clinically significant. Hypoechoic nodules distributed diffusely within normal pancreatic parenchyma may be seen as an incidental finding on ultrasound examination. However, nodular hyperplasia cannot be differentiated from malignant pancreatic neoplasia, which can appear similar on ultrasound examination. A biopsy is need for a definitive diagnosis.
Insulinomas (islet cell tumors) are suspected in some animals with hypoglycemia. These tumors generally are small (about 1 cm), focal, hypoechoic nodules which may be located in any part of the pancreas. Because they are easily missed, a negative ultrasound exam should not rule out insulinomas. Metastatic lesions in adjacent lymph nodes or liver may be more reliably imaged.
Pancreatic carcinomas are a differential for any mass lesion within the pancreas. However, ultrasound cannot reliably differentiate neplasia from pancreatitis, pseudocyst, abscess or nodular hyperplasia. A focal mass is probably the most common ultrasound appearance for malignant neoplasia. In cats, it has been reported that the only ultrasound feature unique to malignant pancreatic neoplasia (versus nodular hyperplasia) is the presence of a single mass greater than 2.0cm in at least one dimension. In this study, feline pancreatic neoplasias tended to have a single, larger mass lesion, while nodular hyperplasia tended to have multiple, smaller nodules. However, there is considerable overlap of both ultrasound and radiographic findings, requiring cytology or histopathology for a definitive diagnosis.