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Biliary obstruction can be difficult for the novice sonographer to assess. It should be noted that biliary tree distension can mimic other pathology and tortuosity may be a normal variation, especially in cats (Figure 8.15). In cases where biliary obstruction is suspected, it is best to refer for immediate complete detailed abdominal ultrasound to minimize morbidity and the potential for life‐threatening biliary peritonitis, especially when gallbladder rupture is suspected (see Figure 8.14C,D). In cats, a bile duct greater than 4 mm is considered to be consistent with extrahepatic biliary obstruction (d’Anjou 2008) (see also Figure 39.4). In cases where peritonitis is clinically suspected or needs to be ruled out, an AFAST with an abdominal fluid score should be performed and used to guide fine needle aspiration of peritoneal effusion for fluid analysis, cytology and culture (see Chapter 43).

Figure 8.14. Gallbladder mucocele. (A) Gallbladder mucocele in a dog. Note the echostructural appearance of the gallbladder lumen (GB, gallbladder). In real time, mucoceles are typically seen as immobile biliary patterns that have a stellate or fine striated character referred to as the “kiwi fruit” appearance. Also note the typical concurrent distension of the gallbladder that is enlarged because of the mucocele. These findings should prompt a complete evaluation of the peritoneal cavity by AFAST and assignment of an abdominal fluid score to search for any evidence of bile peritonitis (free fluid) secondary to gallbladder rupture, and a thorough abdominal ultrasound interrogation of the entire biliary tract by an experienced sonographer. (B) Additional example of a gallbladder mucocele demonstrating the variability of their ultrasonographic appearance. (C) Ruptured gallbladder secondary to mucocele. When a kiwi fruit‐like structure (*) is not seen associated with the gallbladder and found in any quadrant of the peritoneal cavity, a mucocele may be eviscerated from the gallbladder and freely floating within the abdomen, as shown in this image. Characterization of the abdominal effusion (anechoic in near‐field) as bile peritonitis further supported this clinical suspicion (FF, free fluid; LIV, caudate lobe of liver; RK, right kidney). (D) Ruptured gallbladder secondary to gallbladder mucocele. If the gallbladder is not visualized, or lacks its normal expected curvilinear contour, gallbladder rupture should be suspected. In this image, the dorsal gallbladder wall is deviated from its expected course. Signs of rupture of the gallbladder can also include loculated echogenic fluid within the gallbladder fossa with adjacent hyperechoic reactive mesentery and/or free‐floating shadowing choleliths. Degrees of bile peritonitis (either loculated within the gallbladder fossa or throughout the abdominal cavity) should be present and an AFAST with the assignment of an abdominal fluid score should be performed. Accessible fluid should be aspirated to characterize its nature and direct surgical intervention. Discontinuity of the dorsal aspect of the gallbladder is marked with cursors [<<<]. In cases of gallbladder rupture, emergent exploratory surgery is indicated. Additional, less common rule‐outs for lack of visualization of the gallbladder include gallbladder agenesis (which is rare but has been reported) and obstruction of the cystic duct (such as with a mass lesion), that is impairing normal gallbladder and bile duct filling. Moreover, obscured gallbladder visualization may occur with emphysematous cholecystitis or severe cholelithiasis, warranting the use of radiography.

Figure 8.15. Biliary tract distension. (A) Cat with bile duct tortuosity which can be a normal variant and common finding. This variant can be easily mistaken for a variety of pathology (nodules, cysts, masses) by the novice or hasty sonographer. Careful correlation with other clinical findings is imperative for an accurate assessment (CBD, common bile duct; GB, gallbladder). (B) A cat with suppurative cholangitis with concurrent pathological tortuosity and dilation of the bile duct. The bile duct is seen ventral to the portal vein and can be distinguished from vessels with color flow Doppler evaluation. In cats, a bile duct greater than 4 mm is considered to be consistent with extrahepatic biliary obstruction. When the biliary duct wall is irregular or unevenly thickened, neoplasia should be a differential. (C) Cholelithiasis can be seen anywhere along the biliary tract and presents challenges for the novice sonographer. This image demonstrates a hyperechoic cholelith at the level of the duodenal papilla in a dog. Note the strong distal acoustic shadow (clean shadow) and lack of normal architecture of the duodenal papilla (duod, duodenum). This example emphasizes the need for a complete detailed abdominal ultrasound evaluation by a veterinary radiologist or specialist with advanced ultrasound training.