Diffuse liver disease is a worldwide problem. The causes are several, with non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease, and viral B or C hepatitis being the most frequent. No matter what the cause is, the chronic inflammation of the liver and the cellular death lead to liver tissue scarring, namely liver fibrosis, that may progress to cirrhosis with its complications.
Staging liver fibrosis is important for the management and prognosis of diffuse liver disease. For decades, liver biopsy has been the reference standard for the staging of liver fibrosis.
Shear wave elastography (SWE) is a method able to assess the tissue stiffness by applying a mechanical stress that induces the generation of shear waves, which then propagates into the tissue with a speed that is proportional to the stiffness of the tissue. The shear waves are generated by a body-surface compression, as in transient elastography (TE), or by the push-pulse of a focused ultrasound beam, as in acoustic radiation force impulse (ARFI) techniques.
The speed of the shear waves is related to the stiffness: they travel faster in stiffer tissue. Using a formula and making some assumptions, it is possible to convert the speed into units of stiffness, ie kilopascals.
A fibrotic tissue is harder (stiffer) than a normal tissue, and an increase of fibrosis is coupled with an increase of the stiffness. Therefore, there is a close positive relationship between fibrosis and stiffness.
TE is an SWE technique performed with the FibroScan system (Echosens). This system has a probe with a tip at the end and a button on the lateral part of it. By pushing the button, the tip compresses the body surface and this deformation propagates into the liver as shear waves. An ultrasound beam tracks the shear wave speed and sends information back to the software of the system. The final reading is in kilopascals. The FibroScan quantifies the stiffness but doesn’t assess the morphology of the liver.
The ARFI techniques are implemented in ultrasound systems that are used for other diagnostic purposes when a patient with diffuse liver disease is evaluated. In fact, using an ultrasound system, it is possible to study the organ’s morphology with B-mode, the hemodynamics with Doppler, and to characterize focal liver lesions with contrast agents. ARFI techniques make use of the energy of the ultrasound beam to generate the shear waves whose speed propagation is assessed in m/s: higher the speed stiffer the tissue.
ARFI techniques include point shear wave elastography (pSWE) and two-dimensional shear wave elastography (2D-SWE). pSWE measures the stiffness in a small and fixed region of interest whereas with 2D-SWE the stiffness is obtained over a large field of view and a color-coded image, from which the stiffness value is gotten, is displayed on the monitor of the ultrasound system. The shear wave speed can be converted into kilopascals; the ultrasound systems generally provide both speed values in m/s and stiffness values in kilopascals.
The stress is made directly into the liver; therefore, the examination can be performed also in patients with ascites.
All the published studies have shown that the ARFI techniques have accuracy similar to or higher than FibroScan for the staging of liver fibrosis. Over the last years, the assessment of liver stiffness with SWE techniques, either TE or ARFI, has increasingly been used as a means to noninvasively staging liver fibrosis. Currently, guidelines have accepted that SWE techniques can safely replace liver biopsy in several clinical scenarios. SWE can safely be used also in children. It is feasible in children of all ages and has many pediatric applications in the setting of chronic liver disease.
Bibliography
- Barr RG, Wilson SR, Rubens D, Garcia-Tsao G, Ferraioli G. Update to the Society of Radiologists in Ultrasound Liver Elastography Consensus Statement. Radiology 2020; 296:263–74.
- Ferraioli G, Wong VW, Castera L, Berzigotti A, Sporea I, Dietrich CF, Choi BI, Wilson SR, Kudo M, Barr RG. Liver Ultrasound Elastography: An Update to the WFUMB guidelines and recommendations. U Med Biol 2018; 44:2419–2440.
- Ferraioli G. Review of liver elastography guidelines. J Ultrasound Med 2019; 38:9–14.
- Ferraioli G, Barr RG, Dillman JR. Elastography for pediatric chronic liver disease: a review and expert opinion. J Ultrasound Med 2020; doi: 10.1002/jum.15482
Giovanna Ferraioli, MD, FAIUM, is a researcher at Medical School University of Pavia, Italy. She’s the lead author of WFUMB guidelines on liver elastography, co-author of the SRU consensus, and of several international guidelines on elastography.
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