S. Boroffka1, Dr med vet, PhD, DECVDI
Diagnostic imaging plays an important role in the diagnosis of hepatic disease. When a definitive diagnosis is made, a liver disease is treatable with a predictable prognosis. With the clinicopathological examination, hepatic damage and dysfunction is identified and characterized and causes may be identified for primary or secondary liver disease. In this lecture, different imaging modalities, such as radiography, ultrasonography (US), computed tomography (CT), magnetic resonance imaging (MRI), and scintigraphy, with their advantages and disadvantages, are being discussed.
Patients with liver disease are often a diagnostic challenge, since they show varied and vague clinical signs. The liver has a great functional reserve capacity, so significant hepatic dysfunction (≥ 55%) has to occur before patients show symptoms. It is also sometimes difficult to decide whether liver enzyme elevations are significant and whether they represent primary or secondary liver disease.
This lecture will focus on radiography and ultrasonography (US), using CT and MRI in some patients for illustration.
On survey radiography the liver size can be assessed but no information on the parenchymal structure, besides mineralization's or gas pattern, can be achieved. Ultrasonography allows noninvasive detailed assessment of the internal architecture of the liver and the adjacent structures, including the portal vein, enabling identification of the various forms of hepatic disease in small animals.
It is important to realize that a tentative diagnosis of primary hepatic disease can be made only by correlating the ultrasonographic abnormalities with the history, physical examination findings, and clinical laboratory results. However, for definitive diagnosis of specific hepatopathies, often histopathological examination of the liver biopsy specimens is required.
Anatomy of the Liver
In both cats and dogs, the liver consists of six liver lobes: right medial and lateral, left medial and lateral, and quadrate and caudate lobes. The gallbladder is located between the right medial and quadrate lobes.
The liver is drained by the hepatic lymph nodes, which lie along the portal vein at the porta hepatis. They are multiple in both dogs and cats and are rarely seen in normal adult patients. Sometimes, the drainage route may pass through the gastric or splenic lymph nodes before drainage to the hepatic nodes, so in cases of suspected neoplasia, all these lymph nodes should be examined with ultrasound (or another modality) to look for metastasis.
Radiography remains the mainstay of abdominal pathology assessment in dogs and cats. No radiographic changes alone are pathognomonic for one liver pathology against another, but radiography is unique in offering a quick and cheap overview of the abdomen. Thorax radiographs should be considered if neoplasia or systemic disease is suspected. In patients with ascites, radiographs are of less value.
The liver lies between the diaphragm and the stomach, so on a lateral radiograph, the hepatic size can be evaluated by changes of the stomach position. The "gastric axis" is the line drawn between the center of the fundus dorsally and the ventral and caudal part of the stomach (the antrum/pylorus) and is influenced by the conformation of the patient. Barrel-chested dogs have a gastric axis parallel with the last ribs and deep-chested dogs have a gastric axis perpendicular to the spine. In patients with hepatomegaly the axis is displaced caudally, whereas in microhepatica it is displaced cranially. The tip of the liver should be sharp and pointy and may extend the costal arch. In hepatomegaly, the liver extends beyond the costal arch with rounded margins. The liver should be of uniform soft-tissue opacity and the adjacent falciform fat determines the margins. The diaphragm cannot be identified as a separate structure from the liver, only in patients with a pneumoperitoneum. Mineralizations of either the hepatic parenchyma (neoplasia, chronic inflammation) or in the biliary tree (choleliths or gall bladder wall mineralization) may be seen occasionally. Radiolucencies seen in the liver indicate the presence of gas, such as in the biliary tree indicating reflux, emphysematous cystitis (particularly seen with patients with diabetes mellitus), or hepatic abscesses or necrosis, such as in lobar torsion or entrapment or adenocarcinoma.
The liver is examined using sagittal and transverse probe orientations immediately caudal to the xiphoid and angling dorsally along the ribcage to the left and the right. The depth should be set so the curved reflective interface of the diaphragm is seen in the far field and the time gain compensation adjusted showing a uniform echogenicity at all depths. The ventral falciform fat can be very attenuating of the ultrasound beam and be confused with abnormal liver parenchyma. With the improvement of US image quality, the cystic bile duct (which becomes the common bile duct once the last hepatic duct has joined) can often be followed in cats (< 4 mm) and dogs (< 3 mm), to the major papilla with ultrasound. In cats, the bile duct often appears tortuous at the gall bladder neck.
The major papilla is identified as a homogenous, echogenic round structure in the wall of the duodenum in both normal dogs and cats. The common bile duct passes adjacent to the pancreas, so pancreatic disease can compress and even obstruct the duct. In cats, the pancreatic duct joins the distal common bile duct to have a single opening; in dogs, the pancreatic duct and the common bile duct have separate openings at the major papilla.
Hepatic Diffuse Disease
Ultrasonography alone is not accurate in distinguishing different types of generalized liver diffuse parenchymal diseases resulting in either in hyperechogenicity or hypoechogenicity changes to liver echotexture. It can be difficult to recognize, so comparison of echogenicity to the spleen and kidney (providing these are normal) is the most helpful technique. The spleen is hyperechoic to the liver and the renal cortex is mildly hypoechoic to the liver structure, but the actual echogenicity may vary between patients and transducer settings. It is important to compare echogenicity using the same gain factors. Diffuse liver changes may be accompanied by changes in liver size (usually enlargement) and may also lead to rounding of the liver lobe margin. Some diffuse liver changes may also show mixed hyper- and hypoechogenicity, most likely reflecting severe chronic changes leading to fibrosis, with patches of more regenerative normal liver in between. When diffuse changes are identified, a definitive diagnosis is often only reached by liver biopsy. A fine-needle aspiration (FNA) sample is maybe diagnostic in lymphoma or hepatic lipidosis, but in most patients non-diagnostic.
Hepatic Focal Disease
Ultrasonography is very sensitive in identifying focal lesions, but is not very specific, so cannot distinguish lesion type: nodular hyperplasia, neoplasia (primary and metastatic), and abscesses. Focal liver changes, such as nodules and masses, may be hypo-, iso-, or hyperechoic to the surrounding liver parenchyma. Mineralization, fat, and gas are usually hyperechoic, while hematomas and necrosis are likely to be hypoechoic. For definitive diagnosis of focal liver abnormalities, a FNA or liver biopsy are necessary. Target lesions (ring around a central region) are often associated with malignancy, but may also occur in nodular hyperplasia and other benign liver pathologies.
Anechoic masses with distal enhancement are compatible with hepatic cysts (in cats, these have been associated with polycystic kidney disease) and with biliary cystadenoma (more rarely cystadenocarcinoma).
Liver biopsy is performed either with ultrasound guidance (using a dedicated biopsy needle) or with laparoscopic or surgical techniques only if the coagulation panel is normal.
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