Introduction

The liver can be a challenging organ to interpret on diagnostic imaging. The anatomy of the liver is complex, and many different pathologic mechanisms can create similar imaging features, which can complicate their interpretation, resulting in nonspecific findings.

Normal Anatomy

The liver is an asymmetric organ that can be subdivided into left, central, and right divisions, each containing two lobes. The left division is subdivided into left lateral and left medial lobes. The central division is subdivided into quadrate and right medial lobes. The right division is divided into right lateral and caudate lobes. The caudate lobe is further subdivided into the caudate and papillary processes [1, 2]. Other references describe the liver as composed of four lobes, four sublobes, and two processes [3]. The right division is larger than the left; the hepatic lobes are separated by relatively deep fissures that are more pronounced in the left division. These fissures are difficult to visualize on imaging studies of normal dogs, and can complicate the location of hepatic lesions [4].

The primary blood supply to the liver is provided by the portal vein, which enters at the porta hepatis and provides 75–80% of normal hepatic blood flow [3]. The primary intrahepatic portal tributaries are the right, left, and central branches, which can also help to determine relative lobar anatomy. The hepatic artery also enters the liver at the porta hepatis and provides the remaining 20–25% of hepatic blood flow [3]. The liver is drained by the hepatic veins, which converge at the level of the diaphragm, slightly to the right of midline, to drain into the caudal vena cava. Special attention should be paid to the hepatic vasculature in cases of congenital or acquired portosystemic vascular anomalies.

The anatomy of the biliary system has been described [3]. Hepatocytes produce bile, which drains into bile canaliculi that ultimately form interlobular and lobar ducts, which are intrahepatic. The hepatic ducts represent the extrahepatic extension of the lobar ducts, and enter the excretory tree at the level of the cystic duct which is an extension of the gallbladder. The gallbladder is located ventrally between the quadrate lobe and the right medial lobe, with a fundus, body, and neck in the dog [3]. The neck is considered the beginning of the biliary duct system, extending from the gallbladder neck to the aforementioned junction of the hepatic ducts from the liver. The bile duct continues caudally to enter into the duodenum at the major duodenal papilla.

Characterizing Hepatic Abnormalities

The characteristic features of hepatic disease are described using Roentgen signs, or modifications thereof, as mentioned in the introductory chapter. Once evaluation of the liver is performed, and conclusions are made, differentials should be listed and prioritized using your organizational scheme of choice (DAMN IT V or CITIMITV as mentioned in Chapter 1). While all Roentgen features are useful, an initial approach to evaluation of the liver often begins with assessment of size.

Deciding if the liver is enlarged or normal in size can be determined by comparison to adjacent structures, specifically the costal arch and the gastric axis (Figure 23.1). On radiographs, the caudoventral margin of the canine liver is generally described as ending at the level of the costal arch. However, this is dependent on the radiographs being expiratory and the breed of dog being evaluated. As a general principle, the observation of organ displacement is a window into the origin of masses in the abdomen. Specifically, then, the gastric axis can also be used to evaluate hepatic size. The liver and stomach are loosely connected by a portion of the lesser omentum, the hepatogastric ligament. Therefore, caudal gastric displacement can serve as an indicator of hepatic enlargement. The gastric axis serves as an imaginary line that bisects the dorsal extent of the fundus and the ventral extent of the pylorus on a lateral projection. Generally, on a lateral abdominal radiograph, the gastric axis in the canine and feline patient is parallel to the ribs and perpendicular to the thoracic spine. On a ventrodorsal projection, the gastric axis can be represented as a line connecting the left lateral aspect of the gastric fundus to the right lateral aspect of the pylorus. These landmarks create a line that is typically located at the 10th to 12th intercostal space. The stomach itself is curvilinear, with the gastric body often curved caudally.

Relationship to the costal arch and alterations in the gastric axis can be effective tools to assess for changes in hepatic size. However, some breed and conformational variations exist. In deep‐chested dogs, the caudal margin of the liver may be cranial to the costal arch and the gastric axis may be shifted cranially, resulting in an acute angle with the thoracic spine. In barrel‐chested dogs, the normal liver may extend beyond the costal arch and the gastric axis may be slightly more caudal. In addition, differences in recumbency have been shown to create variation in these measurements, with right lateral recumbency resulting in larger relative measurements than left lateral recumbency in normal dogs and in dogs with hepatomegaly [5].

Similar anatomic guidelines can be used when assessing hepatic size on computed tomography (CT) examinations. Extension beyond the costal arch and gastric displacement can be good indicators of changes in hepatic size. Sonographically, changes in hepatic size relative to adjacent anatomy can be more subjective. Extension beyond the costal arch can be assessed during the examination, and displacement of the stomach can be evaluated with experience.

Fortunately, enlargement often also creates a shape change. Rounded hepatic margins may be the only feature of hepatic enlargement in some patients, and should be used as a strong indicator of hepatomegaly. Rounded hepatic margins can be identified on radiographs, CT, and ultrasound (US).

While many disease processes can overlap with regard to their imaging features, determining if hepatic enlargement is generalized, focal or multifocal can be a useful tool in narrowing the list of potential differentials. Making this distinction also involves assessment of hepatic margins. A focal rounded margin often suggests the presence of a mass, whereas a multifocal lobular or irregular margin may suggest many nodules, or multiple masses.

Some diseases result in a small liver (microhepatia) (Figure 23.2). As with enlargement, determination of hepatic size can generally be assessed by comparison to adjacent anatomy, specifically the costal arch and the gastric axis, with attention to breed variations. As with enlargement, evaluation of the shape and margins can help with generating differentials. Characterizing the margins as well defined and sharp versus lobular and irregular is valuable in differentiation of some congenital diseases from acquired diseases such as cirrhosis and nodular regeneration.

The radiographic opacity of the liver is generally soft tissue. The liver is soft tissue attenuating on CT, and shows coarse but uniform echotexture on US. However, some diseases may change the opacity of the liver. Increases in opacity generally arise from the biliary tree rather than the hepatic parenchyma itself. Hepatolithiasis, or the presence of intrahepatic bile duct stones, can result in the identification of intrahepatic mineral. Because these stones are located in the arborized intrahepatic bile ducts, this mineral often can appear as curvilinear regions of mineral arranged in a branching pattern, depending on severity. Alternatively, these may be multifocal regions of mineral with no discernible pattern to their distribution.

Similarly, gas may contribute to decreased opacity of the liver. While not common, cases of gas identified within the portal vasculature have been reported, often in response to gastric pneumatosis, or in cases of gastric dilation and volvulus (Figure 23.3). This will also result in the impression of arborized, branching regions of gas within the hepatic parenchyma. Focal accumulations of gas could also be present in the case of hepatic abscess, without arborization, but this has not been reported [6].

Ultrasound is an accessible tool for cross‐sectional evaluation of the liver. While the conformation of some patients can preclude evaluation of the entire liver, ultrasound does provide information regarding the parenchyma, which can be characterized as normal, hypoechoic (darker than normal), hyperechoic (brighter than normal), and heterogeneous (nonuniform). These can be further combined to describe more complex changes to the hepatic parenchyma, with terms like heterogeneously hyperechoic used to reflect a nonuniform, coarse increase in hepatic echogenicity. In addition, ultrasound can be used to evaluate hepatic vasculature and the biliary tree.

Computed tomography affords nearly unparalleled ability to evaluate the entire liver, including vascular structures. Attenuation and contrast enhancement of the liver can be measured in Hounsfield units. Alterations in attenuation and enhancement patterns have been correlated with specific disease processes in some cases, though some features remain nonspecific. The evaluation of hepatic attenuation and contrast enhancement during dual‐ and triple‐phase acquisitions can be useful in characterizing hepatic lesions and masses.

Generalized Hepatomegaly

As mentioned above, while there are multiple tests for determination of hepatic size, in practice this determination is largely subjective. Generalized hepatomegaly is characterized on radiography, US, and CT often by a subjective assessment of the margin of the liver (rounded vs sharp); relationship with the costal arch, with careful consideration of breed and conformation; location of the gastric axis; and potential displacement of other abdominal organs (Figure 23.1).

Many diseases causing hepatopathy can result in generalized hepatomegaly, making this a relatively nonspecific finding that must be interpreted in the context of other data, including patient signalment, physical examination findings, clinicopathologic abnormalities, and additional diagnostic testing results. Table 23.1 lists some causes of generalized hepatomegaly categorized using the DAMN IT V system. It is evident that the observation/conclusion of generalized hepatomegaly without additional information is not particularly helpful in narrowing the list of differentials.

Ultrasound Characteristics of Generalized Hepatomegaly

Sonographically, generalized hepatomegaly is similarly subjective and nonspecific. As a generalization, inflammatory and infectious processes and toxicity may result in a generalized reduction in echogenicity. For example, leptospirosis and babesiosis often result in a generalized hepatic hypoechogenicity along with mild, generalized hepatomegaly. Toxins may cause acute hepatic necrosis, which may cause no sonographic abnormalities in the acute phase but have been reported to produce a mild reduction in hepatic echogenicity in the subacute phase.

Category	Etiology
Degenerative/developmental	Nodular hyperplasia Storage disease (i.e., glycogen) Polycystic liver disease
Anomalous/autoimmune
Metabolic	Vacuolar hepatopathy (diabetes mellitus, hyperadrenocorticism) Extramedullary hematopoiesis
Neoplastic/nutritional	Lymphoma Mast cell tumor Malignant histiocytosis Leukemia Diffuse metastasis
Inflammatory/infectious/iatrogenic/idiopathic	Acute hepatitis (i.e., leptospirosis, babesiosis) Fibrosis/early cirrhosis
Trauma/toxic
Vascular	Congestion (right‐sided congestive heart failure, Budd–Chiari syndrome)

Conversely, metabolic diseases such as diabetes mellitus and hyperadrenocorticism or exogenous corticosteroid administration result in glycogen storage and hepatic steatosis, which manifests as diffusely increased hepatic echogenicity.

Heterogeneous echogenicity can also be observed, and is often associated with nodular hyperplasia, extramedullary hematopoiesis, or metastatic neoplasia.

CT Characteristics of Generalized Hepatomegaly

Attenuation refers to the relative reduction in x‐rays as they pass through the patient. Attenuation is analogous to opacity on radiographs but CT has significantly greater contrast resolution, allowing for greater discrimination of tissue types. For example, fluid and soft tissue can be differentiated on CT, yet have the same opacity on radiographs. A full discussion of the physics of attenuation and calculation of attenuation coefficients is beyond the scope of this text, and the reader is directed to textbooks of medical physics.

Attenuation values in some organs can provide useful information when evaluating hepatic pathology. While generalized increases in hepatic attenuation have not been reported, a generalized reduction in hepatic attenuation has been reported in a case of a dog with hyperadrenocorticism [7]. In this case, hepatic attenuation values were negative, and correlated with histopathologic analysis consistent with hepatic lipidosis [8]. Reductions in hepatic attenuation in cats have also been reported in cases of fasting‐induced hepatic lipidosis, but negative attenuation values were not observed [8]. In addition, further evaluation of cats with naturally occuring hepatic lipidosis did not suggest great utility in the use of attenuation values as an aid to diagnosis.

Generalized hepatomegaly can also result from dissemination of hepatic nodules from either metastatic neoplasia or nodular hyperplasia. The discrimination of benign and malignant etiologies is challenging, and is discussed below.

Focal Hepatomegaly

Recall that enlargement of an organ typically causes displacement of surrounding organs. This places further emphasis on the importance of normal radiographic anatomy and normal anatomic variants. Generalized enlargement tends to cause relatively uniform displacement of the gastric axis, while focal enlargements may result in focal displacement of the pylorus, fundus or possibly the gastric body (Figure 23.4a,b). In addition, focal enlargements involving the caudate lobe may result in caudal displacement of the right kidney. Therefore, it is critical to evaluate the location of all organs and identify any displacement. With regard to the liver specifically, reviewing imaging tests for focal displacement of the gastric fundus, body or pylorus can be helpful in identifying focal hepatic enlargement.

Focal masses can result from a multitude of etiologies. Table 23.2 summarizes some causes of focal hepatic enlargement. In general, primary hepatic neoplasms can be hepatocellular, biliary, neuroendocrine, or mesenchymal in origin [9]. The most common primary hepatic neoplasms in dogs are hepatocellular, whereas the most common primary hepatic neoplasms in cats are bile duct tumors [9]. Again, these would be prioritized based on additional information regarding patient signalment, physical examination findings, clinicopathologic abnormalities, and the results of other diagnostic testing.

Ultrasound Characteristics of Nodules and Focal Hepatomegaly

The sonographic appearance of masses is variable, and discriminating between benign and malignant etiologies is challenging. In an investigation comparing the ability of US and CT to predict malignancy, the only significant sonographic feature that predicted malignancy was the presence of cavitation [10]. The presence of cavitation was proposed to reflect regions of necrosis often found in malignant neoplasm. However, CT was considered more accurate than US for predication of the nature of hepatic masses. Hypoattenuation with a threshold of 37 HU also helped predict malignancy, and was certainly influenced by the presence of cavitation. This further supports the concept that relative heterogeneity in both US and CT is suggestive of malignancy. In addition, larger masses were more often malignant, with an average diameter of approximately 8 cm compared to 4 cm in benign masses (Figure 23.5a‐e).

Hepatic abscesses have features of focal hepatomegaly on both ultrasound and radiographs. Sonographically, hepatic abscesses are variable, and can appear hyperechoic, hypoechoic, or heterogeneous [6].

Category	Etiology	Comments/Notes
Degenerative/developmental	Nodular hyperplasia Cyst	Less likely to have capsular structure Fluid attenuation on CT
Anomalous/autoimmunea	N/A
Metabolic	N/A
Neoplastic/nutritional	Adenoma Hepatocellular carcinoma (dogs) Hemangiosarcoma Biliary carcinoma Biliary adenoma	Diffuse enhancement similar to NH
Inflammatory/infectious/iatrogenic/idiopathic	Abscess
Trauma/toxic	Liver lobe torsion Hematoma Arterioportal fistula	Dysplastic arteries shunt directly into portal veins
Vascular	High flow (Congenital) vascular malformationsa	Early in the process, the arterialization of the portal vein may form a focal mass. Over time, this high‐flow lesion results in congestion and ascites

^a Congenital anomalies of the portal venous system are described in the Vascular section.

CT, computed tomography; N/A, not applicable; NH, nodular hyperplasia.

Target lesions (Figure 23.6

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