Imaging Essentials | Small Animal Abdominal Ultrasonography | Liver & Gallbladder: Part 1
Danielle Mauragis, AS, CVT, and Clifford R. Berry, DVM, Diplomate ACVR
University of Florida
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. This article reviews the normal ultrasound appearance of the liver and gallbladder, steps for obtaining appropriate views, and some common abnormalities seen with ultrasonography of these organs. In Part 2, causes of generalized hepatic changes and the ultrasound appearance of biliary abnormalities will be reviewed.
NORMAL ULTRASOUND APPEARANCE
Complete evaluation of the liver requires extensive examination of all aspects of the liver and may also necessitate both standard transabdominal and intercostal (between the ribs within the intercostal space) approaches. Evaluation consists of longitudinal (sagittal and parasagittal) and transverse images of all aspects of the liver (right side, midline, and left side of the liver).
Initial Liver Evaluation
- Clip the hair and apply acoustic gel at a level just caudal to the xiphoid process.
- With the patient in dorsal or lateral recumbency, place the probe just caudal to the xiphoid process.
- Begin the examination with the transducer in a long-axis plane and angle it cranially to view the midsection of the liver.
The liver is composed of 4 lobes, 4 sublobes, and 2 processes (Figure 1):
- Right lobe, including the lateral and medial sublobes
- Quadrate lobe
- Left lobe, including the medial and lateral sublobes, which makes up ⅓ to ½ of the liver
- Caudate lobe, including the caudate and papillary processes.
The hepatic lobes cannot be distinguished with ultrasonography. However, when significant abdominal effusion is present, the divisions between the hepatic lobes become more apparent. In dogs, the liver’s left lobe (medial sublobe), quadrate lobe, and right lobe (medial sublobe) encircle the gallbladder. The caudate lobe extends dorsally and caudally, ending with the renal fossa, which contains the cranial pole of the right kidney.
The liver parenchyma has a coarse echotexture, and portal vessels are the dominant vascular structure noted throughout the liver (Figure 2). The hepatic and portal veins form specific divisions to each of the liver lobes. The porta hepatis is the central portion of the liver to the right of midline where the portal vein, hepatic artery, and bile duct (see Gallbladder Evaluation) enter and exit the liver.
- The portal vein is the easiest structure of these 3 to identify.
- The hepatic artery is smaller and easier to identify using color flow imaging.
- The bile duct is not seen in dogs, but may be visible in cats (up to 3 mm in diameter) in the porta hepatis region.
The portal veins are the dominant vessels in the hepatic parenchyma. They have an outer hyperechoic wall due to fibrofatty connective tissue surrounding the wall and within the wall itself. The intrahepatic portal veins are a continuation of the portal vein proper as it enters the porta hepatis:
- As the main portal vein enters the liver, the right divisional branch diverges from the vein
- The portal vein continues cranially, with the central divisional branch diverging
- The portal vein then continues as the left divisional branch into the left lobe of the liver.
The hepatic veins drain dorsally and to the right into the caudal vena cava (Figure 3). They can be seen as hypoechoic tubular structures that do not have hyperechoic walls; the vessels taper toward the periphery of the liver and enlarge centrally within the liver. The hepatic veins enter into the caudal vena cava in the dorsal right liver along the ventral and lateral wall of the caudal vena cava.
Further Liver Evaluation
1. With the transducer in the subxiphoid position, angle it to the left to evaluate the left lobe in the long-axis plane, with the diaphragm–lung interface (a bright hyperechoic line) noted along the cranial border of the liver (Figure 2). The fundic portion of the stomach is just caudal (Figure 4) to the left lobe of the liver.
2. Angle the transducer back to midline and then toward the right side of the patient to image the gallbladder (Figure 5).
3. Angle the transducer back to midline, and rotate it 90 degrees, with the notch pointing toward the patient’s right side. Then angle the probe ventrally and dorsally to see the entire extent of the liver.
Dividing the liver into thirds ensures evaluation of all 3 sections: right side, midline, and left side (Figure 6). Laterally and caudally, the liver extends to the level of the spleen on the left; dorsally, the liver extends to the level of the right kidney on the right (Figure 7).
Fat within the falciform ligament is seen in the near field, particularly in cats. Often the fat is isoechoic to the liver, and it can be difficult to delineate between the hepatic parenchyma and the falciform ligament (Figure 8).
A “window” that is often used to evaluate dogs and cats with suspected portosystemic shunts is a transverse imaging plane between the dorsal right 11th and 12th intercostal spaces. This window allows visualization of the relationship between the porta hepatis and the aorta, caudal vena cava, and portal vein (Figure 9).
The gallbladder is located to the right of midline. The volume of bile noted within the gallbladder is variable. Fasting and anorexia result in gallbladder distension. In cats, the gallbladder can be bilobed (Figure 10).
Parts of the gallbladder that are not normally visualized are the:
- Gallbladder wall (< 1 mm) (Figure 11)
- Intrahepatic bile ducts
- In dogs, the cystic and bile ducts are not visible; however, feline cystic and bile ducts can be visualized sometimes, and can measure up to 3 mm in diameter each (Figure 12).
Hepatic abnormalities can be categorized as focal, multifocal, or generalized. Focal and multifocal abnormalities (Table 1) are further described by:
- Size: Nodule versus mass (> 3 cm)
- Echogenicity: Anechoic, hypoechoic, hyperechoic, and/or heteroechoic.
Generalized abnormalities will be discussed in Part 2 of this article.
Focal and multifocal nodules are < 3 cm, and can be anechoic, hyperechoic, hypoechoic, and/or heteroechoic, with a target lesion appearance (Figure 13).
Target lesions are nodules with hyperechoic centers and hypoechoic rims. These lesions are often metastatic, particularly if there are multiple target lesions in several abdominal organs (liver, spleen, kidneys). Due to the overlap in ultrasound appearance between benign and malignant lesions, either cytology or histology is needed for a final diagnosis.
Hepatic masses can be complex, with hyperechoic and hypoechoic features and areas of cavitation consistent with necrosis (Figure 14). These lesions can become quite large and create a cranial abdominal mass effect on abdominal radiographs. A large hepatic mass is most commonly a primary hepatic tumor, with varying grades of malignancy (from hepatoma to hepatocellular carcinoma).
Cystic Adenomas & Adenocarcinomas
In cats, cystic adenomas and adenocarcinomas are common and have a hypoechoic appearance with multifocal cystic areas (Figure 15). Other tumors that can cause single or multiple hepatic masses include lymphoma, histiocytic sarcoma, and hemangiosarcoma. Any large tumor mass in the liver can result in hemorrhage and peritoneal effusion.
Mineralization & Intraparenchymal Gas
Other causes of focal or multifocal abnormalities include areas of mineralization (hyperechoic focus with distal shadowing) or intraparenchymal gas (hyperechoic focus with distal acoustic reverberation artifact).
Dystrophic mineralization typically occurs within the parenchyma and can be seen as:
- Chronic granulomas from prior parasitic migration or fungal infection
- Dystrophic mineralization within the biliary tree, secondary to ascending inflammation and cholecystitis (Figure 16)
- Focal mineralized choleliths in the intrahepatic biliary tree.
Intraparenchymal gas can be seen:
- In the biliary tree and gallbladder after surgery
- In the gallbladder wall in diabetic dogs with emphysematous cholecystitis
- Within the biliary tree, secondary to ascending infection
- In a liver abscess and liver lobe torsion (Figure 17).
Ultrasonography of the liver is complicated by the facts that the liver is:
- The largest organ in the abdomen
- Cranial to the stomach
- Protected by the caudal ribs.
Ultrasonography of the gallbladder is complicated by the:
- Small size of the organ
- Echogenic material within it that varies, depending on the species and the patient (Figure 5).
Liver evaluation takes time, with both intercostal and intraabdominal approaches often needed. It is important to note both the structures that can, and cannot, be visualized in the liver and gallbladder (Table 2).
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