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The liver is composed of six lobes: left medial and lateral lobes, quadrate lobe, right medial and lateral lobes, and caudate lobe. Sonographically, the distinction between liver lobes is poorly defined. The left liver is largest and may comprise up to half of the liver mass. The gallbladder contacts the quadrate and right medial lobes. The left liver can be seen lateral and to the left of the gallbladder, and medial to the gastric fundus. The caudate process of the caudate lobe contacts the right kidney at the renal fossa (see Figure 8.1A,B). In general, it is difficult to visualize the divisions between liver lobes unless peritoneal fluid is present (see Figure 8.2E). Falciform fat can be imaged in the near‐field at the ventral liver margin (see Figures 8.2A–D, 8.10C and 39.2). A hyperechoic (bright white) line is imaged cranial to the liver. While this is commonly referred to as the diaphragm, it is actually the interface of the muscular soft tissue diaphragm and the gas‐filled lung (seeFigure 8.2B–E). When this echogenic curvilinear interface is not present or there is interruption of the normal curvilinear continuity of the structure, then congenital or acquired diaphragmatic herniation should be ruled out (see Figure 8.2F,G).

Evaluation of the size of the liver is subjective and is poorly assessed with ultrasound due to variability in patient conformation. Hepatic size estimation is better evaluated on abdominal radiographs. Normal liver margins are sharp and not rounded (see Figure 8.2A,B,E).

Normal hepatic parenchyma is homogeneous and uniform in echogenicity with medium echotexture (coarser echotexture than the spleen) (see Figure 8.1C,D). Echogenicity is generally evaluated in comparison with falciform fat, the right renal cortex and spleen (see Figures 8.1A,B and 8.2A–D). Falciform fat must be distinguished from hepatic parenchyma and is generally isoechoic or hyperechoic to normal hepatic parenchyma (see Figure 8.2A–D and compare to Figure 8.10D), with a coarser echotexture. The liver is hypoechoic (darker) compared to spleen and the left liver can often easily be compared to the head of the spleen whereas the right liver is easily compared to the right kidney, which allows for comparisons at the same image field of view at the same depth, focus and gain (see Figure 8.1C,D). The liver is normally isoechoic to mildly hypoechoic and coarser in echotexture compared to the renal cortex (see Figure 8.1A,B). Subtle changes in echogenicity of the hepatic parenchyma can be difficult to appreciate for the novice sonographer. Using the split‐screen B‐mode function can be helpful for comparing liver, renal and splenic echogenicity comparisons at the same depth, focus, and gain when these organs cannot be imaged in the same frame on nonsplit‐screen B‐mode scanning.

Pearl: Remember the “SLiCK” pneumonic regarding canine echogenicity: the spleen (S) is hyperechoic (brighter) to liver (Li) which is slightly more hyperechoic (brighter) or isoechoic (same echogenicity) to the cortex of the kidney (CK). In the order of most to least echogenic, this spells “SLiCK.” For the feline, hepatic echogenicity is often compared to the adjacent falciform fat and the feline liver should be isoechoic to slightly hypoechoic to the falciform fat.

Evaluation of hepatic vasculature should generally be reserved for the more experienced sonographer. However, distension of the hepatic veins is fairly easy to identify and can be learned by the novice. Hepatic veins are distinguished from portal veins based on the echogenicity of the walls. Portal veins have bright echogenic walls whereas the walls of the hepatic veins are not visualized (see Figure 8.10A,B). The hepatic veins can be traced through the hepatic parenchyma to the caudal vena cava to confirm their identity. The caudal vena cava traverses the dorsal hepatic parenchyma slightly to the right of midline, and the hepatic veins empty into the cava near the diaphragm (Figure 8.3A,B). See also Chapters 7, 20, 26, and 36.

Figure 8.3. Hepatic venous congestion. The differentiation between hepatic veins and portal veins can be challenging for the novice. However, imaging the caudal vena cava as it passes through the diaphragm in the longitudinal plane is easy to learn and helps accurately identify distended hepatic veins by their associated branching into the caudal vena cava. Normally, hepatic veins are inconspicuous. Shown in (A) and (B) is the same example of moderate hepatic venous (HV) congestion unlabeled and labeled. Note how the hepatic veins can be seen emptying into the caudal vena cava (CVC) prior to the CVC passing through the diaphragm (Diaphragm →). The image was acquired at the TFAST diaphragmatico‐hepatic (DH) view. (C) Another example of HV congestion being traced to the caudal vena cava (CdVC) near the diaphragm (Diaphragm →). Detecting hepatic venous distension is clinically helpful and aids in the detection of right‐sided volume overload, right‐sided heart failure or an obstructive venous lesion between the liver and right atrium, which would then prompt further investigation and help guide management. (D) A similar correlative figure with overlays to show the anatomy of the area.

Source: (B) and (D) courtesy of Dr Gregory Lisciandro, Hill Country Veterinary Specialists and FASTVet.com, Spicewood, TX, and (C) courtesy of Dr Terri DeFrancesco, Raleigh, NC.

The gallbladder is surrounded by hepatic parenchyma on the right side of the liver. The size is variable depending on when gallbladder contraction last occurred. In the typical normal state, the gallbladder is an anechoic structure with a uniformly thin echogenic wall. Normal gallbladder wall thickness is less than 1 mm in cats and less than 3 mm in dogs (Hittmair et al. 2001; Spaulding 1993); however, for all intents and purposes, the gallbladder wall should be a thin white line in both species (Figure 8.4A–C; see also Figure 8.11). Gallbladder sludge (echogenic gravity‐dependent material within the lumen) is common in dogs and generally a benign finding in the absence of other significant ultrasonographic and clinical findings although the presence of “normal” gallbladder sludge has been more recently questioned (Tsukagoshi et al. 2012) (see Figure 8.12A–D). The finding may be an indication of cholestasis. The intrahepatic bile ducts are not seen unless there is extrahepatic biliary obstruction of at least five days’ duration when the fluid dilated ducts will have the appearance of hepatic veins (without the hyperechoic walls of portal veins) (see Figure 8.15A,B). If available, color flow Doppler is useful for differentiating the lack of flow in a dilated biliary duct from the normal blood flow in hepatic and portal vessels; however, tracing structures in real time is often adequate. The common bile duct is small and difficult to see in the normal state in dogs and cats, but is generally easier to identify in the cat.

Figure 8.4. Normal gallbladder and variations. (A) The normal gallbladder wall (not labeled) is inconspicuous with normal thickness <1 mm in cats and <3 mm in dogs (GB, gallbladder). However, normal wall thickness does not rule out mild, low‐grade or chronic inflammation. (B) Similarly labeled image showing a normal canine gallbladder wall. Note the anechoic structure is marginated ultrasonographically by a thin hyperechoic (bright white) line. Evaluation of the margin of the gallbladder in real time provides a global view of the gallbladder wall and can aid in determining gallbladder wall integrity. The curvilinear line in the far‐field is created by the interface between the diaphragm and lung (air). (C) Image of a bilobed gallbladder may be seen in both dogs and cats but is more common in cats, as was seen in this normal feline patient (GB, gallbladder).

Pearl: It may be difficult to tell the difference between a dilated biliary tract (no flow) and hepatic veins (flow), in which case color flow Doppler may be helpful.

Artifacts are commonly encountered when performing the POCUS liver and gallbladder examination and include mirror image, acoustic enhancement, side‐lobe, slice thickness, and edge shadowing. These are discussed in more detail in Chapter 3 and 5.

 Mirror image. The most common of these artifacts results in a mirror image of the liver and gallbladder being seen on the far side of the lung–diaphragm interface. This should not be mistaken for a diaphragmatic hernia (see Figure 8.9D).

 Acoustic enhancement. Acoustic enhancement is associated with a fluid‐filled gallbladder that causes an increase in echogenicity of the hepatic parenchyma on the far side of the gallbladder. Acoustic enhancement should not be mistaken for increased liver echogenicity.

 Side‐lobe and slice thickness. These artifacts involve fluid‐filled organs like the gallbladder and give the false impression of the presence of luminal sediment.

 Edge shadowing. This artifact also involves fluid‐filled organs like the gallbladder and gives the appearance of a defect in the wall that can be mistaken for gallbladder rupture or gallbladder wall mineralization.

Pearl: Common artifacts in the liver include mirror image, acoustic enhancement, side‐lobe, slice thickness, and edge shadow.