Imaging in Hepatic Disease in Small Animals
Routine abdominal radiographs are useful to determine liver size and may detect irregular liver borders. Mineralized densities that involve parenchyma or the biliary tree can reflect stasis of bile flow, dystrophic mineralization associated with congenital malformations, acquired duct “sacculation,” chronic duct inflammation, or choleliths. Choleliths that contain enough calcium bilirubinate or calcium carbonate are radiographically visible. A mass effect in the right cranial quadrant in suspected EHBDO may represent an engorged gallbladder, pancreatitis, neoplasia, or focal bile peritonitis. Radiographic suspicion of abdominal effusion (poor abdominal detail) may prompt diagnosis of bile peritonitis and ascitic effusion. Gas within hepatic parenchyma or biliary structures indicates an emphysematous process (eg, cholecystitis, choledochitis, infected biliary cyst, hepatic abscess, necrotic tumor mass) and warrants prompt antimicrobial therapy and either surgical intervention or percutaneous, ultrasound-guided aspiration/lavage. Thoracic radiography can indicate signs of systemic disease (eg, metastatic lesions, pleural fluid). Finding sternal lymphadenopathy is common in cats with the cholangitis/cholangiohepatitis syndrome, in which it reflects hepatic inflammation.
Although cholecystography can be accomplished with iodinated contrast given PO or IV, contrast radiographic imaging of the biliary system is rarely pursued. Distribution and concentration of contrast agents within biliary structures is influenced by numerous variables, including hyperbilirubinemia and major duct occlusion. At best, these agents may disclose choleliths, polyps, or sludged bile but are insufficient to confirm bile peritonitis or to localize the site of leakage. Multisector CT and/or hepatic ultrasonography are more useful to discern these processes.
Contrast studies of the portal vasculature are the gold standard for confirmation of a congenital portosystemic shunt. Radiographs should be taken in right and left lateral and ventrodorsal positions for best test sensitivity. Multisector CT imaging produces exceptional images and has replaced radiographic portography for diagnosis of congenital portosystemic shunts because it allows contrast injection into a peripheral vessel, can capture numerous images per second, and allows three-dimensional anatomic reconstruction.
There are many diagnostic applications of hepatic ultrasonography: 1) identify distention and determine thickness of biliary structures; 2) verify common bile duct obstruction; 3) detect gallbladder mucoceles and cholelithiasis; 4) differentiate between diffuse and focal hepatic abnormalities; 5) identify and determine dimensions of “mass lesions”; 6) identify pancreatic, mesenteric, and perihepatic lymphadenomegaly; 7) in conjunction with vascular studies, identify congenital intrahepatic and extrahepatic portosystemic vascular anomalies (PSVAs), APSSs, arteriovenous malformations, and hepatic venule distention reflecting passive congestion; and 8) detect small volume abdominal effusion and small volume of fluid surrounding the gallbladder. However, although abdominal ultrasonography has become an indispensable diagnostic tool to assess the liver and biliary system, its use is highly operator dependent, and findings must always be reconciled with the history, physical examination findings, and clinicopathologic data. Reconciliation of data is best done by the principal clinician managing the case, who has the most knowledge of the animal's management and prognosis.
Multisector CT imaging, available in specialty referral practices and university teaching hospitals, can distinguish mass lesions, detect changes in structure of hepatic parenchyma and the biliary system, identify choleliths, detect abnormal hepatic perfusion (involving the portal vein, hepatic artery, or hepatic vein), and portal thrombi, and can detail the extent of traumatic hepatobiliary injuries.