where they join the caudal vena cava (see Figures 8.3A,B and 39.3). Moderate to marked dilation of the hepatic veins is usually secondary to right‐sided cardiac insufficiency, including right‐sided volume overload in patients receiving intravenous fluid therapy (see Chapters 7, 20, 26, and 36). Obstruction of the caudal vena cava due to thrombosis, stricture (kinking) or neoplasia can also cause marked distention of the hepatic veins (Kolata et al. 1982; Crowe et al. 1984; Lisciandro et al. 1995; Fine et al. 1998). Hepatic venous distension should be evaluated in conjunction with other clinical signs, and congestion with concurrent ascites (modified transudate) further supports the presence of right‐sided heart failure or obstruction of the hepatic caudal vena cava (see also Chapters 7, 20, 26, and 36).
Gallbladder
The gallbladder is easily imaged with ultrasound because it is fluid filled. The gallbladder is generally round to oval; however, it may be bilobed, particularly in cats, and this is generally of no clinical consequence (see Figure 8.4C). Several artifacts are common when scanning the gallbladder, due to its fluid‐filled nature, and the novice sonographer should become familiar with these artifacts in order to minimize misinterpretations. These include acoustic enhancement, side‐lobe artifact, edge shadowing artifact, and mirror image artifact (when the gallbladder contacts the diaphragm–lung interface).
Gallbladder Wall
Evaluation of the gallbladder wall is generally easily accomplished due to the contact between the anechoic bile and the more echogenic gallbladder wall and surrounding hepatic parenchyma. The normal gallbladder wall appears as an echogenic (white) line surrounding the luminal contents (see Figure 8.4A–C). Diffuse gallbladder wall thickening may occur with a wide range of conditions such as acute or chronic cholecystitis, hepatitis, canine anaphylaxis (Quantz et al. 2009), right‐sided heart failure, iatrogenic volume overload (Nelson et al. 2010), and hypoalbuminemia (Nyland et al. 2002) (see Chapters 7, 20, 26, and 36). Therefore, the finding must be interpreted in conjunction with additional ultrasound findings as well as clinical signs and biochemical alterations (Figure 8.11; see also Figures 7.11, 7.12, 18.22, and 39.6).
Generally, gallbladder wall thickening is nonspecific for any of the above conditions and thickening is typically associated with wall edema with similar sonographic features despite the underlying pathologic cause (see Figure 8.11 and Table 7.5). Most commonly, the gallbladder wall appears diffusely hypoechoic with parallel hyperechoic lines on either side, referred to as a “double rim effect” or “halo effect” (Nyland et al. 2002; Quantz et al. 2009) (see Figures 7.11, 7.12, 18.22, and 39.6). This change needs to be distinguished from small‐volume fluid external to the gallbladder, where effusion within the gallbladder fossa can mimic the double rim or halo effect. Focal and/or irregular thickening of the gallbladder wall is less common, and may be consistent with chronic cholecystitis or neoplastic change. A flaccid or undulating gallbladder wall may be consistent with wall rupture and warrants correlation to the index of suspicion for biliary peritonitis as well as clinical and biochemical assessment of the patient.
Pearl: In patients in which peritonitis is suspected (ruptured gallbladder, perforated bowel, low‐grade bleed), especially in dehydration, fluid resuscitation and reevaluation (serial exam within the next 2–4 hours) with AFAST and an abdominal fluid score often prove most helpful because once rehydrated, peritoneal effusion develops or progresses (higher abdominal fluid score) and sampling for fluid characterization becomes possible.
Gallbladder Lumen
Evaluation of the luminal gallbladder is typically straightforward due to the ease of scanning through a fluid acoustic window.
Biliary sediment or sludge is common in dogs and can be easily recognized (Figure 8.12). In general, the finding of gravity‐dependent, mobile material is an incidental finding but could be an indication of cholestasis. Gallbladder wall abnormalities should be correlated to clinical and/or clinicopathological signs of hepatobiliary disease changes (Tsukagoshi et al. 2012). Luminal sediment is usually hyperechoic (bright) and nonshadowing. When mineralized, the debris may cast a distal acoustic shadow (see Figure 8.12D). Biliary sludge is classified as gravity or nongravity dependent and mobile or consolidated. Nongravity‐dependent material may further be classified as adherent or nonadherent to the gallbladder wall.
Calcified material or choleliths can occasionally be observed within the biliary tract and may also be an incidental finding. These gallbladder stones will cause a distal acoustic shadow similar to a urolith (Figure 8.13; see also 8.12D).
Pearl: When suspicion is raised for intraluminal gallbladder contents, the gallbladder can be reexamined at the end of the study to see if the possible intraluminal pathology has settled into gravity‐dependent regions.
Gallbladder mucoceles always have significant implications and their presence should be confirmed by an experienced sonographer as surgery is often indicated. Gallbladder mucoceles have a distinct appearance and, in the mature form, have a stellate (“kiwi fruit‐like”) appearance caused by fracture lines between mucous collections (Figure 8.14A,B). When immature, there are variable degrees of nonmobile sludge seen between the focal collections of mucus. In addition, the gallbladder wall may be thickened, irregular and hypoechoic or hyperechoic due to wall inflammation, and may lead to wall necrosis and gallbladder rupture (see Figure 8.14C,D). Six ultrasonographic patterns of mucoceles have been described: type 1, immobile echogenic bile; type 2, incomplete stellate pattern; type 3, typical stellate pattern; type 4, kiwi fruit‐like pattern and stellate combination; type 5, kiwi fruit‐like pattern with residual central echogenic bile; and type 6, kiwi fruit‐like pattern. Based on one study, there was no correlation between the ultrasonographic pattern and clinical disease status or gallbladder rupture (Choi et al. 2014).Figure
