The Liver part 3: The Gall Bladder
Following on from the previous articles examining ultrasonography of the liver parenchyma and associated pathological conditions, this article will focus on the gall bladder and biliary system.
Examination of the gall bladder and biliary system should be included as part of the routine ultrasonographic study of the liver and surrounding cranial abdominal anatomy. The indications for examination of the gall bladder and patient preparation are the same as those described for the liver.
Normal anatomy and ultrasonographic appearance
The gall bladder is situated in a fossa between the quadrate and right medial lobes of the liver. It acts as a reservoir for bile and appears as an anechoic round/oval to teardrop shaped structure situated just to the right of midline in most patients. The gall bladder can be bilobed in some cats.
Gallbladder size
The size of the gall bladder is variable and will alter depending on when the animal was fed. Post-prandially enzymes are released which cause the gall bladder to contract. The volume of the gall bladder is increased in anorexic or fasted patients.
In the normal gall bladder, the wall appears as a smooth and thin hyperechoic line (Figure 1). Wall thickness is less than 1mm in cats and 2-3mm in dogs1.
Figure 1. This transverse plane image shows the normal gall bladder (GB) in a dog. Note the acoustic enhancement (*) making the liver parenchyma deep to the gall bladder appear more echoic.
The intrahepatic biliary tree is not visible in normal patients. Biliary canaliculi merge to form the interlobar and lobar ducts which eventually form the hepatic ducts which join the cystic duct from the gall bladder, thereby becoming the common bile duct (Figure 2). In normal dogs, the common bile duct can be difficult to visualise, while in cats it is often more visible in normal patients.
The normal common bile duct is less than 3mm diameter in dogs and less than 4mm diameter in cats2. In dogs, the common bile duct joins the duodenum at the major duodenal papilla, separately from the pancreatic duct. An accessory pancreatic duct joins the duodenum at the minor papilla in most dogs though anatomic variation exists. In cats, the common bile duct fuses with the pancreatic duct before entering the major papilla. Only 20% of cats are estimated to have an accessory pancreatic duct3 (Figure 3).
Figure 2. A schematic representation of the anatomy of the biliary system and surrounding structures.
Figure 3. A schematic representation of the pancreatic ducts and biliary system in dogs and cats. PD = Pancreatic Duct, AP= Accessory Pancreatic Duct.
Biliary sludge is seen routinely within the gallbladder lumen of asymptomatic dogs. Sludge appears as hyperechoic, non-shadowing sediment and is usually mobile. This causes sludge to alter location based patient position and it will collect on the dependant aspect of the gall bladder as a result of gravity (Figure 4). The presence of biliary sludge may indicate delayed gall bladder emptying4 and in cats has been found to be predictive of increased liver enzymes5.
Figure 4. Gall bladder sludge (sediment) – The image on the left shows hyperechoic sediment within the lumen of the gall bladder (red arrow). The right image shows a layer of sludge within the gall bladder lumen (S) with more mineralised material adjacent to the caudal wall of the gall bladder (red arrowhead). Note the acoustic shadowing distal to the mineralised material (*).
Gall Bladder Wall Thickening
Thickening of the gall bladder wall can be caused by several different conditions (Figure 5):
- Cholecystitis/Cholangiohepatitis
- Inflammation of surrounding anatomy such as hepatitis or pancreatitis
- Wall oedema caused by hypoproteinaemia, right sided congestive heart failure, portal hypertension or biliary obstruction
- Cystic hyperplasia
- Gall bladder polyps
- Neoplasia
Figure 5. Gall bladder wall thickening – In this image of the gall bladder the wall is diffusely thickened with a double layer appearance (red arrows) consistent with wall oedema.
Cholelithiasis
Choleliths can be an incidental finding within any part of the biliary tract. The calculi are variable in size and shape and will appear hyperechoic, often with associated acoustic shadowing (Figure 6).
They can form secondary to a number of conditions:
- Cholangiohepatitis/cholecystitis
- Biliary stasis
- Dietary factors
- Altered bile composition
The formation of choleliths or choledocholiths (calculi in the common bile duct) can be a primary cause of biliary obstruction. However, they can also form secondary to biliary obstruction.
Depending on their composition, choleliths can be radiopaque. This can allow them to be identified using abdominal radiography.
Figure 6. Cholelithiasis/Choledocholithiasis – This image shows hyperechoic mineralised material within the lumen of the gall bladder neck and common bile duct (red arrow). Distal to the mineralised material, multiple lines of acoustic shadowing can be seen indicating the presence of multiple small mineralised areas.
Gall Bladder Mucocele
Gall bladder mucocele is an important cause of icterus and biliary obstruction. Seen most commonly in older small to medium breed dogs. It is caused by mucinous hyperplasia leading to excessive mucus accumulation and gall bladder distension which may progress to wall necrosis and rupture.
Ultrasonographically, mucoceles have static, hyperechoic, radiating striations leading to a “Kiwi fruit-like” pattern6 (Figure 7).
Figure 7. Gall bladder mucocele – this image shows a distended gall bladder with immobile hyperechoic striations seen in association with a mucocele. (Image courtesy of Dr Sally Griffin, Willows Referrals. Not for reproduction).
Biliary Obstruction
Causes of biliary obstruction can be categorised as intrahepatic or extrahepatic, and include:
- Neoplasia of the liver and/or gallbladder
- Mucocele
- Cholelithiasis/choledocholithiasis
- Neoplasia of the surrounding anatomy especially the pancreas and/or duodenum
- Inflammation of the surrounding anatomy especially the pancreas and/or duodenum
- Local abscessation
- Local lymphadenopathy
Obstruction can lead to common bile duct and gall bladder distension. However, gall bladder distension was seen in less than 50% of cats with extrahepatic biliary obstruction7.
In chronic obstruction, progressive dilation will cause hepatic duct distension. Following 5-7 days of obstruction the hepatic ducts can be visualised on ultrasonographic imaging as anechoic, tortuous tubes closely associated with the portal veins. Doppler ultrasonography can be useful to differentiate the dilated hepatic ducts from vascular structures because they exhibit no flow compared with the neighbouring portal vasculature (Figure 8). Biliary tract dilation can remain present for some time following relief of any obstruction. Therefore, the presence of biliary tract dilation should always be considered along the history and clinical presentation.
Figure 8. Hepatic duct dilation – this image shows dilation of the intrahepatic biliary system. By examining the structures using colour flow doppler, no doppler shift will be registered within the dilated biliary structures (red arrow). In comparison, the neighbouring vascular structures have blood flow and are highlighted by the colour pixels within the colour doppler sample gate. (Image courtesy of Dr Sally Griffin, Willows Referrals. Not for reproduction).
Cholecystocentesis
Ultrasound-guided percutaneous cholecystocentesis can be used to obtain biliary samples for culture and cytology or to facilitate drainage of the gall bladder in cases of extrahepatic biliary obstruction. Taking samples via this technique has been shown to be relatively safe8, 9.
A transhepatic approach using a 22-gauge needle is preferred, in order to isolate any bile leakage from the gall bladder.
References
- Hittmair K.M., Vielgrader H.D., Loupal G. (2001) Ultrasonographic evaluation of gallbladder wall thickness in cats. Veterinary Radiology and Ultrasound 42:149-155.
- Léveillé R., Biller D.S., Shiroma J.T. (1996) Sonographic evaluation of the common bile duct in cats. Journal of Veterinary Internal Medicine 10: 296-299.
- Otte C.M.A., Penning L.C., Rothuizen J. (2017) Feline biliary tree and gallbladder disease: aetiology, diagnosis and treatment. Journal of Feline Medicine and Surgery 19: 514-528.
- Tsukagoshi T., Ohno K., Tsukamoto A., Fukushima K., Takahashi M., Nakashima K., Fujino Y., Tsujimoto H. (2012) Decreased gallbladder emptying in dogs with biliary sludge or gallbladder mucocele. Veterinary Radiology and Ultrasound 53: 84-91.
- Harran N., d’Anjou M.A., Dunn M., Beauchamp G. (2011) Gallbladder sludge on ultrasound is predictive of increased liver enzymes and total bilirubin in cats. Canadian Veterinary Journal 52: 999-1003.
- Besso J.G., Wrigley R.H., Gliatto J.M., Webster C.R.L. (2000) Ultrasonographic appearance and clinical findings in 14 dogs with gallbladder mucocele. Veterinary Radiology and Ultrasound 41: 261-271.
- Gaillot H.A., Penninck D.G., Webster C.R., Crawford S. (2007) Ultrasonographic features of extrahepatic biliary obstruction in 30 cats. Veterinary Radiology and Ultrasound 48: 439-447.
- Schiborra E., McConnell J. F., Maddox T.W. (2017) Percutaneous ultrasound-guided cholecystocentesis: complications and association of ultrasonographic findings with bile culture results. Journal of Small Animal Practice 58: 389-394.
- Byfield V.L., Callahan Clark J. E., Turek B. J., Bradley C.W., Rondeau M. P., (2017) Percutaneous cholecystocentesis in cats with suspected hepatobiliary disease. Journal of Feline Medicine and Surgery 19: 1254-1260.
Further reading
d’Anjou M.A., Penninck D. (2015) Liver, In: Atlas of Small Animal Ultrasonography 2nd edn., Eds: Penninck D. and d’Anjou M.A., John Wiley & Sons, Chichester, pp 183-238.
Larson M. (2018) Liver and Spleen, In: Textbook of Veterinary Diagnostic Radiology 7th edn., Ed: Thrall D., Elsevier, St. Louis, pp 792-822.
Nyland T.G., Larson M. and Mattoon J.S. (2015) Liver, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds: Mattoon J.S. and Nyland T.G., Elsevier, St. Louis, pp 332-399.
Rademacher N. (2011) Liver, In: BSAVA Manual of Canine and Feline Ultrasonography., Eds: Barr F. and Gaschen L., BSAVA publications, Gloucester, pp 85-99.
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