Small Animal Abdominal Ultrasonography, Part 2: Liver & Gallbladder
Welcome to our series of articles on small animal abdominal ultrasonography. The initial articles provided an overview of basic ultrasonography principles and a discussion about how to perform a sonographic tour of the abdomen. This article—and the rest of the series—will discuss ultrasound evaluation of specific abdominal organs/systems, including scanning principles, normal sonographic appearance, and identification of common abnormalities seen during ultrasound examination.
Read the Small Animal Abdominal Ultrasonography articles published in Today’s Veterinary Practice at tvpjournal.com:
- Basics of Ultrasound Transducers & Image Formation (January/February 2015)
- Physical Principles of Artifacts & False Assumptions (May/June 2015)
- Basics of Imaging Optimization—How to Obtain High-Quality Scans (November/December 2015)
- A Tour of the Abdomen: Part 1 (January/February 2016) and Part 2 (March/April 2016).
When using the systematic approach described in previous articles, the sonographic tour of the abdomen begins in the cranial abdomen, evaluating the liver and gallbladder. Proper ultrasound evaluation of the liver includes:
- Volume or size (enlarged or small)
- Margins/borders (smooth versus irregular)
- Overall echogenicity of the hepatic parenchyma
- Appearance of the portal and hepatic veins
- Distribution of any abnormalities (focal, multifocal, or generalized).
Part 1 of this series—published in the May/June 2016 issue of Today’s Veterinary Practice—reviewed the normal ultrasound appearance of the liver and gallbladder as well as the sonographic appearance of nodules. This article reviews abnormalities of the hepatobiliary system found via ultrasonography.
The normal echotexture of the liver is a subjective evaluation. The clinician or technician must know how to improve and manipulate the image in order to present a “normal” liver with the appropriate echogenicity. Too much gain results in increased echogenicity and misinterpretation that the liver is abnormal, whereas too little gain results in decreased echogenicity and misinterpretation that the liver is abnormally hypoechoic (Figure 1).
Diffuse Liver Disease
Diffuse liver disease can be marked by an increase, decrease, or no changes in overall echogenicity (Table 1, Figure 2). In dogs with increased echogenicity secondary to vacuolar hepatopathy, the ultrasound waves can appear hyperattenuating. This occurs when the ultrasound beam no longer penetrates to the depth that would be expected for a given frequency. Typically, the microconvex transducer (C8-5 at 8 MHz) can penetrate to the level of 8 cm. In diseases that cause vacuolar hepatopathies, however, the ultrasound beam is often attenuated to a depth of only 4 to 5 cm.
Other Liver Diseases
Other diseases affect the hepatic parenchyma diffusely but cause no changes in the overall echogenicity of the liver (eg, lymphoma, disseminated mastocytosis, acute hepatitis or cholangiohepatitis). This is why cytology or histology is required for definitive diagnosis.
Changes in hepatic size can be symmetric or asymmetric (Table 2); increased size often results in rounding of the hepatic margins (Figure 3).
- Ill-defined nodular areas of decreased echogenicity and hyperplasia often indicate vacuolar hepatopathy (Figure 4).
- Diffuse heterogeneous enlargement of the liver can be seen as a specific pattern in dogs with hepatocutaneous syndrome (superficial necrolytic dermatitis; Figure 5).
- An overall decrease in echogenicity with an increase in size can be caused by cholangitis or cholangiohepatitis. In these cases, the portal markings appear brighter than usual (Figure 6).
- A hyperechoic liver that can be normal or decreased in size with portal hypertension and ascites indicates hepatic cirrhosis (Figure 7). These dogs often have multiple acquired portosystemic shunts in the region of the normal renal vasculature at the level of the aorta and caudal vena cava (Figure 8).
A Primer on Attenuation & Echogenicity
Attenuation is the loss of acoustic energy or number of ultrasound waves traveling at depth. Hyperattenuation results in fewer ultrasound waves interacting with tissue at depth; therefore, the overall image becomes darker as the clinician looks deeper into a tissue. Hypoattenuation describes areas that do not attenuate the ultrasound waves, resulting in artifact and distal acoustic enhancement, and the area deeper to the cystic structure appears “whiter” on the image.
Echogenicity is the characteristic internal architecture of a given organ that is based on reflectivity of organ parenchyma. Tissues with increased echogenicity are called hyperechoic and are usually represented by increased grayscale or white, while tissues with decreased echogenicity are called hypoechoic and are usually represented by darker, decreased grayscale values. Areas that lack echogenicity—such as fluid-filled structures, including blood vessels or cysts—are called anechoic and typically appear completely black (again, with distal acoustic enhancement due to lack of attenuation of the ultrasound waves through the fluid-filled structure).
The gallbladder is normally found to the right of midline surrounded by the hepatic parenchyma. The cystic and bile ducts are not normally visualized in dogs but can be seen in cats (up to 2–3 mm, Figure 9).
Some echogenic material may be seen within the canine gallbladder (Figure 10). In addition to wall thickening, echogenic material in the gallbladder is not normal in cats and indicates inflammatory biliary disease, such as cholecystitis (Figure 11). Other luminal abnormalities include nonmineralized and mineralized choleliths (Figure 12).
Mucoceles are an important cause of hepatobiliary disease in dogs. A mucocele is an abnormal collection of bile salts and mucus within the gallbladder that may potentially cause hepatobiliary obstruction, gallbladder wall necrosis, and rupture (Figure 13).
The pathogenesis of mucoceles is unknown, although multiple factors have been suggested to result in abnormal bile salt retention, decreased biliary motility (gallbladder contractility), and excessive mucus secretion by the biliary epithelium. Dogs with hyperadrenocorticism have a 29-fold higher risk for developing a mucocele than those without hyperadrenocorticism.1
The ultrasound features of a mucocele include variations of mucus collections and ultimate linear striations (stellate or kiwi-like appearance), with the gallbladder completely filled with echogenic material. These striations are secondary to fracture lines between the mucus collections. The gallbladder wall is typically thick and the gallbladder abnormally distended.
Gallbladder wall necrosis leads to leakage of bile contents into the peritoneal cavity, with an increase in echogenicity to the mesentery, which is in contact with the gallbladder wall (Figure 14). The abnormal mucus collection can extend into the cystic and bile ducts, resulting in extrahepatic biliary obstruction. Mucoceles have been reported in dogs with no clinical signs; however, mucoceles progress, with the possibility of future wall necrosis and perforation, which should be considered a reason to monitor the lesion or pre-emptively surgically remove the mucocele.
Extrahepatic Biliary Obstruction
Extrahepatic biliary obstruction in dogs usually results from pancreatitis. Inflammation and edema surround the bile duct, causing obstruction at the level of the pancreas. In experimental bile duct obstructions, the bile and cystic ducts dilate within 24 hours. The gallbladder distends within 48 hours, although it might take a week before the intrahepatic ducts become dilated.
The distended extrahepatic ducts are usually tortuous and can be distinguished easily from the portal vein using color Doppler ultrasound. The dilated intrahepatic ducts can be seen around the portal veins within the hepatic parenchyma (Figure 15). Intrahepatic bile ducts have abrupt changes in luminal diameter, irregular walls, and branching patterns when compared with the portal vasculature (tapering luminal diameter, smooth walls, and branches in the midzone to the periphery of the hepatic parenchyma).
Other causes of extrahepatic biliary obstructions include choleliths, duodenal strictures at the major duodenal papilla, and biliary tumors (Figure 16).
Cats can develop a condition known as triaditis, which involves concurrent cholecystitis/cholangiohepatitis, pancreatitis, and inflammatory bowel disease.
When performing ultrasonography of the liver and gallbladder, it is important to realize that a negative scan does not rule out disease. In particular, hepatic scans can appear normal in dogs and cats with certain round cell tumors, such as lymphoma and systemic mastocytosis. Cytology or histology is required for definitive diagnosis in patients in which these tumors are suspected. Biliary disease is common in dogs and less so in cats. It is incumbent on the novice sonographer to review current textbooks and other sources for further descriptions detailing the ultrasound appearance of hepatic and biliary disorders.
- Mesich ML, Mayhew PD, Pack M, et al. Gallbladder mucoceles and their association with endocrinopathies in dogs: A retrospective case-control study. J Small Anim Pract 2009; 50:630-635.
Kremkau FW. Sonography Principles and Instruments, 8th ed. Philadelphia: Saunders Elsevier, 2010.
Mattoon J, Nyland T. Small Animal Diagnostic Ultrasound, 3rd ed. Philadelphia: Elsevier, 2015.
Penninck D, d’Anjou M (eds). Atlas of Small Animal Abdominal Ultrasonography, 2nd ed. Ames, IA: Wiley Blackwell Publishing, 2015.