Liver Elastography: Basic Principles, Evaluation Technique, and Confounding Factors

*Felix Bende and Tudor Moga*

## **Abstract**

Ultrasound-based elastography techniques have received considerable attention in the last years for the noninvasive assessment of tissue mechanical properties. These techniques have the advantage of detecting tissue elasticity changes occurring in various pathological conditions and are able to provide qualitative and quantitative information that serves diagnostic and prognostic purposes. For liver applications and especially for the noninvasive assessment of liver fibrosis, ultrasound-based elastography has shown promising results. Several ultrasound elastography techniques using different excitation methods have been developed. In general, these techniques are classified into strain elastography, which is a semi-quantitative method that uses internal or external compression for tissue stimulation, and shear wave elastography, which measures the ultrasound-generated shear wave speed at different locations in the tissue. All liver elastography techniques have a standardized examination technique, with the patient in a supine position, while the measurements are performed through the right liver lobe. There are also some confounding factors that need to be taken into account when performing liver elastography such as a higher level of aminotransferases, infiltrative liver disease, liver congestion, cholestasis. This chapter briefly introduces the basic principles of liver elastography and discusses some important clinical aspects of elastography, such as the examination technique and the limitations.

**Keywords:** liver ultrasound elastography, shear wave elastography, transient elastography, acoustic radiation force impulse, strain elastography, liver fibrosis

#### **1. Introduction**

Chronic liver disease is a major health problem worldwide. This situation is generated by a wide range of chronic liver injuries such as chronic viral hepatitis, chronic alcohol abuse, non-alcoholic fatty liver disease, autoimmune hepatitis, primary biliary cirrhosis, and other less frequent causes. Regardless of the liver disease etiology, a common pathway of fibrosis is set up, which progresses and leads to liver cirrhosis that may be complicated by portal hypertension, liver failure, and hepatocellular carcinoma.

The evaluation of patients with chronic liver disease must be as simple as possible, cost efficient, and easily repeatable. While the liver biopsy is still considered the gold

standard for liver fibrosis evaluation, due to its shortcomings (invasiveness, potential complications, inter-/intra-observer variability, sampling error) [1–3] scientific and practical interest has focused on the development of noninvasive techniques for the diagnosis of liver fibrosis.

Elastography can be used to assess liver fibrosis noninvasively. It measures the tissue behavior when mechanical stress is applied, either using ultrasound (ultrasoundbased elastography) or magnetic resonance (magnetic resonance elastography).

Ultrasound elastography is perhaps the most important breakthrough in the evolution of ultrasound in the last 20 years. The basic idea behind liver elastography is that the elasticity of the tissue examined offers information on liver health. A stiffer liver tissue usually indicates the presence of chronic liver disease.

Mainly, most liver ultrasound elastography techniques are based on the principle of measuring the speed of the shear wave that propagates through the liver which is influenced by the stiffness of the tissue. The speed of the shear wave is proportional to the tissue stiffness. Basically, the stiffer the liver, the faster the shear wave will propagate through the liver.

The value of ultrasound-based elastography for staging chronic liver disease has been established by numerous studies [4–7]. Moreover, its value for evaluating and predicting chronic liver disease complications (portal hypertension, hepatocellular carcinoma) has been also proven in different studies [8–10].

The European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) and the World Federation of Societies for Ultrasound in Medicine and Biology (WFUMB) have issued guidelines and recommendations on the clinical use of ultrasound-based elastography and describe in detail their basic principles [11–13].

This chapter focuses on the basic principles of elastography, which is an important aspect for every clinician or practitioner who is performing or learning liver elastography. Moreover, clinical features such as the examination techniques of different liver elastography methods and the factors that influence the liver elastography results are described and discussed.

#### **2. Basic principles**

Elastography assesses tissue elasticity, which is the tendency of tissue to resist deformation with an applied force or to resume its original form after removal of the force. Elastography can be considered a type of remote palpation that allows the measurement and display of the biomechanical properties in a tissue that acts against the shear deformation. Shear deformation is generated by applying a force either to a single location or broadly across the body surface. A force can be applied by vibrating the body surface that produces a natural internal physiological motion or using the ultrasound transducer to create focused acoustic radiation force at controlled depths [13, 14].

All ultrasound-based elastography methods use ultrasound to measure the tissue shear deformations resulting from an applied force. The type of force applied can be quasi-static or dynamic. Quasi-static forces do not allow the acquiring of images that are quantitative for tissue properties. Dynamic forces allow the quantification of the tissue properties. They include impulses that can be produced mechanically at the body surface or by acoustic radiation force impulse at controlled depths.

According to the EFSUMB guidelines [11], elastography techniques can be classified according to how the displacement data are shown. Three options are available as follows:

*Liver Elastography: Basic Principles, Evaluation Technique, and Confounding Factors DOI: http://dx.doi.org/10.5772/intechopen.102371*


For liver applications, elastography methods that display the shear wave speed are the most commonly used in practice, followed by strain and displacement imaging (for liver lesions), which are less frequently used. The elastography methods integrated into clinical practice for the liver are described in **Table 1**.

### **2.1 Strain elastography**

Strain elastography is the most widely implemented elastography method on commercial systems; however, it is the least used technique for liver applications. The force used in strain elastography is either produced with the ultrasound probe or due to the internal physiological motion. The axial displacement images are calculated using radiofrequency echo correlation tracking or Doppler processing, which converts the axial displacement images into strain images [14, 15]. Excitation with manual pressure measures elasticity in superficial tissues. A disadvantage of this excitation method is that manual stress is not efficiently transmitted to deeper tissues. Excitation from natural physiologic motion, such as cardiac pulsation and respiration, is another mechanism of generating tissue stress. Deep organs, such as the liver or the kidney, can be assessed with this method [14, 15].

