Clinical Utility of the Right Intercostal Window for Ultrasonography of the Right Cranial Abdomen
David S. Biller, DVM, DACVR
Professor and Head of Radiology, Kansas State University, College of Veterinary Medicine, Department of Clinical Sciences
Manhattan, KS, USA
For many sonographers, a standard abdominal ultrasound examination is performed by scanning the ventral abdomen from a subcostal approach with the animal in dorsal recumbency. Unfortunately, right cranial abdominal structures, such as the right aspect of the liver, portal hilus (hepatic lymph nodes, common bile duct, caudal vena cava, and portal vein), right limb and body of the pancreas, duodenum, right kidney, and right adrenal gland are often not visible from the standard subcostal approach. Evaluation of right cranial abdominal organs via the subcostal approach may be difficult in dogs that are deep-chested, have microhepatia, have large amounts of gas in the gastrointestinal tract, or have large volume peritoneal effusion.
Access to the liver in dogs may be difficult because portions are located beneath the rib cage. In large and deep-chested dogs, complete evaluation of the liver may require a right lateral intercostal approach from the last 3-4 intercostal spaces.
The right lateral intercostal approach is essential in evaluation of the portal hilus and nearby vessels. Important structures in this region that can be seen include the aorta, caudal vena cava, portal vein, and common bile duct.
The aorta is the most dorsal of the vessels in the region of the portal hilus. It can be found dorsal to the caudal vena cava and the diaphragm. The margin of the diaphragm blends with the ventral wall of the aorta. In some cases, the azygos vein may be seen running parallel and to the right of the aorta.
The caudal vena cava is ventral to the aorta and runs through the foramen venae cavae of the diaphragm to the right of midline in the mid-dorsal diaphragm. It is surrounded by the caudate and right lateral liver lobes. The caudal vena cava can be differentiated from the aorta by the presence of hepatic veins entering.
From the right lateral intercostal window, the portal vein is ventral and to the left of the caudal vena cava and dorsal to the common bile duct.
The common bile duct is ventral and to the right of the portal vein. The common bile duct may or may not be seen, but should be less than 3 mm in diameter in a normal dog. This structure is often not seen from the standard ventral approach because of intestinal gas. With the right lateral intercostal approach, the liver is used as an acoustic window.
Complete evaluation of the pancreas is difficult from the standard ventral abdominal approach. Reasons for this difficulty include intestinal gas and cranial abdominal pain in cases of pancreatitis. The right lateral intercostal window is useful in evaluation of the right lobe and body of the pancreas. From this approach, the right lobe of the pancreas lies ventral to the right kidney, ventrolateral to the portal vein, and dorsal or dorsomedial to the descending duodenum. The body of the pancreas unites the right and left lobes caudal to the pylorus and lies ventral to the portal vein and craniomedial to the right kidney. The colon is caudoventral to the pancreas. The pancreaticoduodenal vein, which drains into the portal vein may be seen in the right lobe of the pancreas, running parallel to the descending duodenum. This vessel is not visible in the other pancreatic lobes. To ensure thorough evaluation of the right lobe of the pancreas, the entire descending duodenum should be followed.
Because of its cranial location, the right kidney is difficult to examine in many dogs from a subcostal approach. It often lies dorsal to small intestine, which can be a barrier if it contains gas. Positioning the dog in left lateral recumbency and placing the transducer dorsally in the intercostal space can allow better access to the kidney. The right kidney can be found by angling or moving the transducer caudally from the portal hilus.
Because of its cranial location, the right adrenal gland is difficult to access from the standard subcostal ventral approach. The level of difficulty is increased when there is gas in the pylorus and the duodenum and in dogs >10 kg. The right lateral intercostal approach is useful for finding this structure. Landmarks for locating the right adrenal include the caudal vena cava, the origin of the celiac and cranial mesenteric arteries, and the craniomedial aspect of the right kidney. To find the gland in long axis, the caudal vena cava is located caudal to the portal hilus.
Multiple lymph nodes can be assessed from the right lateral intercostal window. The hepatic lymph nodes are found next to the portal vein, 1-2 cm caudal to the portal hilus. The left hepatic nodes range from 1 to 6 cm in length and are found in the lesser omentum dorsal to the common bile duct. The right hepatic nodes are smaller and are found next to the body of the pancreas.
The right lateral intercostal approach is useful for diseases involving the right lateral, right medial, and caudate lobes of the liver. This view is especially helpful in diseases that cause microhepatia, such as cirrhosis or congenital portosystemic shunts. Ultrasonographic findings associated with cirrhosis include, microhepatia, hyperechoic parenchyma, irregular margination, ascites, and regenerative nodules. Mass and nodular lesions of the liver may also be evaluated from the right lateral intercostal approach, especially in large and deep chested dogs. These lesions could easily be missed in these dogs if the examination is limited to the subcostal approach.
Diseases involving the portal vasculature are indications for the right lateral intercostal view. This view is indicated in any animal suspected to have a portosystemic shunt. A single extrahepatic shunt is difficult, if not impossible, to identify from the standard subcostal approach, especially in large dogs. The right lateral intercostal view is useful in detection of intrahepatic portosystemic shunts as well.
Portal vein thrombosis, an uncommon condition that is rarely diagnosed in live dogs, can be diagnosed using the right lateral intercostal scan plane.
Diseases of the biliary system are often incompletely evaluated via the subcostal approach. The right lateral intercostal approach is useful in these cases. One indication for this approach is suspected biliary tract obstruction, especially for assessment of the common bile duct.
The right intercostal approach is useful in cases of pancreatitis. These animals are painful, and the intercostal approach avoids the pain induced with scanning from the subcostal approach. The most consistent landmarks for the right pancreatic lobe are the duodenum and the right kidney. The right lobe of the pancreas is found dorsomedial to the duodenum and ventral to the right kidney. With inflammation and bowel gas associated with pancreatitis, vascular landmarks are often not visible.
Diseases of the right kidney and right adrenal gland may be more thoroughly evaluated via the right intercostal view than the standard subcostal approach. This is also a good view for comparison of the echogenicities of the liver and renal cortex.
The hepatic and sometimes the pancreaticoduodenal and gastric lymph nodes can be assessed via the intercostal window. Normal lymph nodes may not be seen because their echogenicities are similar to surrounding fat and muscle. Inflammation and neoplasia will cause lymph nodes to enlarge and become hypoechoic, making them more easily detectable.
Percutaneous ultrasound-guided fine-needle aspirate and biopsy are two commonly performed procedures due to their low cost and relative lack of invasiveness. Using ultrasound guidance, rather than blind techniques, makes it less likely to end up with a non-representative sample. Lesions of the right cranial abdomen might not be accessible from a standard ventral subcostal approach. The right intercostal approach may allow access to these lesions.
In conclusion, a ventral subcostal approach is often inadequate for a complete ultrasonographic examination. The right lateral intercostal scan plane allows for a complete evaluation of the liver, portal hilus, pancreas, duodenum, right kidney, right adrenal gland, and cranial abdominal lymph nodes. Animals in which this approach is most useful are large and deep-chested dogs, dogs with gas in the gastrointestinal tract, cases of microhepatia, peritoneal effusion, and abdominal pain.