Introduction:
Ultrasound elastography, or sonoelastography, is used in clinical settings to assist in diagnosing and managing diffuse liver diseases. Its applications in clinical liver care include assessing fibrosis stage in patients with hepatitis C and B, distinguishing non-alcoholic steatohepatitis from simple steatosis in non-alcoholic fatty liver disease, and prognostic evaluation of autoimmune liver diseases.
Ultrasound elastography encompasses various technologies for estimating tissue stiffness, such as real-time/static elastography (RTE), transient elastography (TE), acoustic radiation force impulse (ARFI) imaging, and real-time shear wave elastography (SWE). These techniques can be broadly categorized into non-imaging and imaging methods, with imaging techniques further divided into strain imaging and shear wave imaging.
What Is Elastography?
Ultrasound elastography (USE), an imaging modality sensitive to tissue stiffness, was initially introduced in the 1990s and has since undergone significant refinement to enable quantitative assessments of tissue stiffness. These methods capitalize on the altered elasticity of soft tissues due to specific pathological or physiological processes.
For example, solid tumors often exhibit mechanical differences compared to surrounding healthy tissues, while fibrosis in chronic liver diseases results in increased liver stiffness. Consequently, elastography techniques can differentiate between affected and normal tissue, aiding in diagnostic evaluations.
Elastography-based imaging techniques have recently garnered considerable attention for their non-invasive tissue mechanical property assessment. By detecting changes in soft tissue elasticity associated with various pathologies, these techniques provide qualitative and quantitative information that is valuable for diagnosis. Measurements are typically obtained using specialized imaging modes that detect tissue stiffness in response to mechanical forces, such as compression or shear waves.
Ultrasound-based methods are particularly notable due to their widespread availability, including bedside use, and relatively low cost. Various ultrasound elastography techniques utilizing different excitation methods have been developed and broadly categorized into strain imaging methods employing internal or external compression stimuli and shear wave imaging utilizing ultrasound-generated traveling shear waves.
While ultrasound elastography has demonstrated promising outcomes for non-invasive liver fibrosis assessment, novel applications are emerging in breast, thyroid, prostate, kidney, and lymph node imaging.
What Are the Technical Protocols for Elastography?
The technical protocols for elastography include:
1. Strict adherence to a protocol is necessary.
2. Patients should fast for at least four hours before the examination.
3. The examination should be conducted with the patient in a supine or slight left lateral position, with the arm raised above the head to enhance the intercostal space.
4. Measurements should be taken through an intercostal approach at the location providing the best acoustical window.
5. Measurements should be performed 1.5 to 2.0 cm below the liver capsule to avoid reverberation artifacts.
6. The optimal location for maximum shear wave generation is 4.0–4.5 cm from the transducer.
7. Ensure the transducer is perpendicular to the liver capsule.
8. Avoid placing regions of interest over large blood vessels, bile ducts, or masses.
9. Select the appropriate transducer for transient elastography based on the patient's body habitus.
10. Obtain ten measurements from ten independent images at the same location, using the median value for transient elastography and point shear wave elastography techniques. When a quality assessment parameter is used, three or five measurements may be appropriate for 2-D shear wave elastography.
11. Use the interquartile range/median (IQR/M) as a quality measure. For kPa measurements, the IQR/M should be <0.3, and for m/s measurements, it should be <0.15 for an accurate dataset.
What Are the Applications of Elastography in Liver Ultrasound?
1. Hepatitis C Viral (HCV) Disease: While only a fraction of HCV-infected patients progress to cirrhosis, accurately identifying those at risk is crucial for treatment allocation. Liver biopsy has traditionally been used for risk stratification, but its drawbacks limit its widespread use. Transient elastography (TE) and shear wave elastography (SWE) offer promising non-invasive alternatives, demonstrating high accuracy in diagnosing significant fibrosis (≥F2) and cirrhosis (F4). Cost-effectiveness studies suggest that integrating annual TE into HCV management could reduce the need for biopsies and increase cirrhosis diagnosis rates, offering substantial benefits in terms of cost and patient outcomes.
2. Hepatitis B Viral (HBV) Disease: Non-invasive methods like transient elastography (TE) and acoustic radiation force impulse (ARFI) imaging show promise in diagnosing significant fibrosis (≥F2) in chronic HBV patients, with comparable accuracy to liver biopsy. TE and ARFI have demonstrated sensitivities and specificities ranging from 70% to 93% and 38% to 92%, respectively, for diagnosing significant fibrosis. Overall, these non-invasive techniques offer valuable diagnostic insights for managing chronic HBV, reducing the reliance on invasive procedures like liver biopsy.
3. Non-alcoholic Fatty Liver Disease (NAFLD): It encompasses two main forms: simple steatosis (SS) and non-alcoholic steatohepatitis (NASH), the latter carrying a risk of progressing to cirrhosis. Accurately distinguishing between these forms is crucial, yet conventional methods like CT and MRI are limited in their ability to do so without resorting to an invasive liver biopsy.
While it remains uncertain whether shear wave elastography (SWE) can detect early-stage NASH, evidence suggests that ultrasound elastography could be valuable in differentiating NASH from SS.
Both transient elastography (TE) and ARFI-based SWE have demonstrated high diagnostic accuracy in differentiating significant fibrosis (≥F2) in NAFLD patients, with sensitivity and specificity ranging from 67% to 88% and 64% to 91%, respectively. ARFI elastography has also shown promise in differentiating NAFLD from NASH or fibrosis, with an accuracy of 0.899 and sensitivity and specificity of 85% and 83.3%, respectively.
4. Alcoholic Liver Disease (ALD): Patients with ALD require an accurate assessment of liver fibrosis for monitoring and treatment planning. Similar to other forms of CLD, ALD is characterized by steatosis, inflammation, and fibrosis, with potential progression to cirrhosis if left untreated. TE and ARFI have shown strong correlations with histological fibrosis staging and high diagnostic accuracy for detecting significant fibrosis and cirrhosis. Additionally, research suggests that liver stiffness measurements may decrease significantly with alcohol abstinence, indicating a potential role for elastography in monitoring patients' compliance with abstinence regimens.
5. Autoimmune Liver Diseases: These include primary biliary cirrhosis (PBC), primary sclerosing cholangitis (PSC), and autoimmune hepatitis (AIH), which often require histological examination for diagnosis and staging. However, elastographic techniques offer promising, non-invasive alternatives for assessing liver fibrosis in these conditions.
Studies have shown that transient elastography (TE) and shear wave elastography (SWE) correlate well with fibrosis stage in PBC and PSC patients, providing accurate staging without needing a biopsy. Longitudinal monitoring with elastography can serve as a prognostic tool, particularly in PSC, where changes in liver stiffness measurements over time can predict adverse outcomes.
6. Focal Liver Lesions: Conventional B-mode ultrasound is cost-effective for screening focal liver lesions in cirrhotic patients but lacks specificity compared to contrast-enhanced techniques. Sonoelastography offers a non-contrast method to differentiate benign from malignant lesions. However, current elastography technologies have limitations: TE lacks imaging capabilities, strain elastography has depth limitations, and SWE may not penetrate deeply enough.
Previous studies on RTE and ARFI elastography showed promising sensitivity and specificity in distinguishing malignant from benign liver lesions. However, subsequent literature needs to be more consistent. While some studies support the efficacy of elastography in lesion characterization, others report significant overlap between malignant and benign lesions, making differentiation challenging.
7. Portal Hypertension: It is a key feature of cirrhosis and is typically diagnosed by directly measuring the hepatic venous pressure gradient (HVPG). HVPG is crucial for predicting varices and complications like ascites, typically occurring at values above 10 mm Hg. However, HVPG measurement is invasive and costly. Liver and spleen stiffness have emerged as biomarkers for portal hypertension.
Transient elastography (TE) shows high sensitivity and specificity for detecting significant portal hypertension. Spleen stiffness, measured with TE, also predicts portal hypertension accurately. Shear wave elastography (SWE) has shown better diagnostic accuracy than TE, particularly for diagnosing clinically significant portal hypertension. A liver stiffness value of 24.6 kPa with SWE offers high sensitivity and specificity for diagnosing portal hypertension.
Conclusion:
Liver elastography offers a valuable, non-invasive tool for assessing liver fibrosis and other liver conditions. With its ability to provide quantitative measurements of tissue stiffness, elastography contributes to improved diagnosis, staging, and monitoring of liver diseases such as hepatitis, fatty liver disease, and cirrhosis. Despite some limitations and the need for adherence to specific protocols, liver elastography shows great promise in enhancing patient care by reducing the need for invasive procedures like liver biopsy and enabling timely intervention to prevent disease progression.
