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Conventional ultrasound findings in chronic liver disease
Introduction
Chronic liver disease (CLD) is a substantial worldwide problem. Any disease that incites liver inflammation can lead to liver fibrosis, which can then progress to cirrhosis.
The stage of liver fibrosis is important to determine prognosis, surveillance, progression, or regression of disease. The process of fibrosis is dynamic, and regression of fibrosis is possible up to the stage of early cirrhosis with treatment of the underlying conditions. Therefore it is important to detect the presence of the disease before the development of decompensated cirrhosis. Worldwide there is a significant increase in fatty liver disease, which can lead to cirrhosis, and noninvasive methods to detect steatosis and monitor progress or regression of the disease are needed.
With increasing fibrosis, the liver becomes stiffer and eventually portal hypertension develops. Previously, the only method of quantifying the degree of fibrosis was a random liver biopsy, which is an imperfect reference standard. Although conventional ultrasound (US) is limited in the detection of fibrosis stages less than cirrhosis, it is a widely used screening examination. Conventional US is used as a screening tool for assessment of steatosis because it is noninvasive, lacks radiation, and is widely available. Other methods such as shear wave elastography are more accurate in the assessment of the stage of fibrosis, and new US techniques are becoming available for fat quantification, which are discussed in other chapters of this book.
For liver stiffness and fat quantification, high quality B-mode images are required for accurate measurements. This chapter reviews the conventional US techniques and how to optimize both B-mode and Doppler images, eliminate artifacts, and use these features in CLD.
The use of B-mode imaging for liver stiffness measurements is discussed in Chapter 4 , portal hypertension in Chapter 8 , and fat quantification in Chapter 11 . Details about Doppler studies for the evaluation of portal hypertension are given in Chapter 8 . Detection and characterization of focal liver lesions in CLD are presented in Chapter 13 . A review of the normal anatomy of the liver can be found elsewhere. ,
B-mode imaging
Optimizing B-mode images
When evaluating patients with suspected CLD, a complete US evaluation of the liver should be performed. The entire liver should be evaluated in two planes to reconstruct its three-dimensionality. In adults the examination should be performed with a curved transducer optimized for abdominal imaging, usually with a range of 2 MHz to 6 MHz. The frequency of the transducer should be adjusted as needed for the patient’s body habitus. Most modern transducers are wideband and have the ability to adjust the center frequency to optimize the imaging. Most systems have a RES (resolution) setting using higher frequencies in easy-to-scan patients, a GEN (general) setting using mid-range frequencies, and a PEN (penetration) setting using lower frequencies in difficult-to-scan patients. Spatial compounding and harmonic imaging also improve the quality of the image by decreasing artifacts and clutter. Using these settings appropriately will improve the image quality. Generally, the left lobe of the liver is best imaged in a supine position using a subcostal approach whereas the right lobe is best imaged using a left lateral decubitus position using both a subcostal approach and through an intercostal window. Having the patient take a deep inspiration may be required to visualize the dome of the liver. A small sector probe also can be helpful in anatomically difficult locations and in pediatric patients. Vascular landmarks should be included in the images to allow identification of the segment according to the Couinaud classification. Color Doppler, power Doppler, or microvascular flow techniques are used to evaluate vessel patency and flow dynamics. Optimization of the Doppler images should be performed, including selecting the appropriate scanning frequency and filters. The biliary system including the gallbladder and intrahepatic and extrahepatic bile ducts also should be evaluated for masses, strictures, or areas of dilation. An examination for CLD should include evaluation of the liver echotexture, evaluation of the liver capsule to assess for nodularity using a high-frequency linear probe, and assessment of the size of the right lobe of the liver and caudate lobe and portal vein diameter. Doppler evaluation with spectral analysis of the portal vein and hepatic veins is required to assess for portal hypertension or hepatic venous outflow obstruction (see Chapter 8 ). Detection and characterization of focal liver lesions should be included as there is a risk for hepatocellular carcinoma in patients with severe fibrosis/liver cirrhosis, which is discussed in Chapter 13 . The spleen size should be estimated and the possible presence of varices should be evaluated, particularly in patients with advanced chronic liver disease (ACLD) as these are signs of clinically significant portal hypertension. Table 2.1 lists the conventional US protocol for CLD.
TABLE 2.1 ■
Typical Scanning Protocol for Patients with Chronic Liver Disease
| Mode | Anatomic Location | Key Findings | |
|---|---|---|---|
|
Curved linear probe |
|
|
| Linear probe | Liver capsule—smooth or nodular | ||
| Color Doppler with spectral analysis |
|
|
|
| Elastography |
|
|
RI , Resistive index.
Fatty liver disease
Diffuse fatty infiltration results in increased echogenicity of the liver, thus the sound transmission is progressively and more markedly reduced from the proximal to the distal portions of the liver. This leads to poor or nonvisualization of the diaphragm, intrahepatic vessels, and posterior portion of the right hepatic lobe. It is important to remember that different US systems may have a more grainy or less grainy appearance of the liver, and learning the appearance of the liver echotexture on the system being used is important.
The main features to take into consideration for a qualitative estimate of liver fat content are liver–kidney contrast, diaphragm definition, and vessel blurring.
Liver–kidney contrast
The echogenicities of liver and kidney are almost equivalent in normal subjects. However, the liver appears hyperechoic as the fat deposition in the liver causes increased echogenicity. The contrast between the liver and parenchyma of the right kidney is therefore increased in liver steatosis ( Fig. 2.1 ). Due to the increased attenuation of the ultrasonic waves in a steatotic liver, the diaphragm, intrahepatic vessels, and the posterior part of the right lobe are poorly or nonvisualized. As an increased liver echogenicity leads to a decreased acoustic impedance between the liver parenchyma and other echogenic structures like portal vein walls, liver capsule, and gallbladder walls, those structures are depicted with difficulty ( Fig. 2.2 ). In rare cases, patients with liver steatosis may show a paradoxical lack of posterior attenuation on US images due to scattering in attenuation of the US beam seen in fatty livers ( Fig. 2.3 ).
Using a combination of some of the aforementioned parameters, the degree of steatosis may be qualitatively assessed as discussed later. , ,
Diaphragm definition
Diaphragm visualization is an important indicator for diagnosing liver steatosis. It can be visualized normally in mild liver steatosis. The echogenicity of the diaphragm is discontinuous in moderate liver steatosis, and it is not visualized in patients with severe liver steatosis.
Vessel blurring
Intrahepatic vessels are sharply demarcated in normal liver. Vessel blurring is an indicator of moderate liver steatosis, and it is based on an impaired visualization of the borders of the intrahepatic vessels and narrowing of their lumen. There are two major reasons for vessel blurring in fatty liver: attenuation of acoustic wave and intrahepatic vascular remodeling. These two reasons can exist alone or at the same time. Intrahepatic vascular remodeling contributes to high resistance and constricted sinusoidal vessels, even to the extent to which intrahepatic vascular shunts develop. Among the various criteria used to diagnose liver steatosis, vascular attenuation, which includes both portal vein and hepatic vein blurring, has lower sensitivity and specificity compared with hepatorenal echo contrast and bright liver.
The assessment of hepatic steatosis on US is subjective, leading to intra- and interreader variability. The features of large hepatic vein blurring, liver–kidney contrast, and overall impression have the highest intrareader, intertransducer, and interreader agreement. Large hepatic vein blurring has the highest accuracy for classifying dichotomized hepatic steatosis grade. Large hepatic vein blurring (intraclass correlation coefficient [ICC]: 0.76), liver–kidney contrast (ICC: 0.78), and overall impression (ICC: 0.75) have the highest mean intrareader agreement, whereas liver echo texture (ICC: 0.43) have the lowest mean intrareader agreement, also depending on the frequency of the transducer. Among individual imaging features, large hepatic vein blurring has the highest diagnostic accuracy (71%) for classifying dichotomized hepatic steatosis (80% sensitivity, 63% specificity) using histological grade as the reference standard. Large hepatic vein blurring provided nominally similar accuracy as overall impression for classifying dichotomized hepatic steatosis with higher specificity but lower sensitivity point estimates.
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