Original Contribution2-D Shear Wave Elastography for Focal Lesions in Liver Phantoms: Effects of Background Stiffness, Depth and Size of Focal Lesions on Stiffness Measurement
Introduction
Two-dimensional shear wave elastography (SWE) is the latest ultrasound (US) elastography technique to provide a 2-D color-coded elastographic map overlaid on a gray-scale anatomic image in real time. It can present quantitative stiffness measurements in meters per second or in kilopascals (kPa) assuming linear elasticity, incompressibility and homogeneity of the elastic medium (Muller et al., 2009, Ferraioli et al., 2012). SWE is considered more objective and reproducible than compression US elastography and has had favorable results for non-invasive diagnosis through measurement of tissue stiffness (Berg et al., 2012, Evans et al., 2012, Liu et al., 2016, Jiao et al., 2017, Suh et al., 2017).
The European Federation of Societies for Ultrasound in Medicine and Biology guidelines and World Federation for Ultrasound in Medicine and Biology guidelines approved liver stiffness measurement using US elastography for assessment of the severity of liver fibrosis in patients with viral hepatitis, alcoholic liver disease and non-alcoholic fatty liver disease (Dietrich et al., 2017, Ferraioli et al., 2018). However, US elastography is not yet recommended for characterization of focal liver lesions and differentiation between benign and malignant liver lesions because of insufficient evidence. Although cutoffs for staging liver fibrosis are US system specific and depend on the etiology of liver disease, available cutoffs for cirrhosis have been proposed (Barr et al. 2015). For focal liver lesions, there have been studies evaluating the cutoff value for differentiation between benign and malignant lesions and assessment of the elasticity of focal liver disease (Guibal et al., 2013, Park et al., 2015, Ronot et al., 2015, Gerber et al., 2017, Grgurevic et al., 2018).
Before approaching the individual SWE value for focal liver lesions, however, it is necessary to clarify the factors associated with target properties that may affect SWE assessment. In superficial organs such as the breast and thyroid, several studies have been published on patient factors or lesion factors that affect SWE assessment of focal lesions. Some researchers have documented that skin thickness, lesion depth and lesion size affect SWE assessment of breast and thyroid lesion properties, such as image quality, SWE color pattern and maximum elasticity (Yoon et al., 2013, Park et al., 2017, Wang et al., 2018). In addition to the depth and size of lesions, it is not known whether the background liver fibrosis affects SWE assessment of focal liver lesions. Therefore, this liver phantom study using 2-D SWE was designed to determine whether quantitative and qualitative SWE results for small inclusions are dependent on background stiffness, depth and size of inclusion and observer.
Section snippets
Phantom design
For the phantom study, two customized phantoms (Project No. 1251-01-00, CIRS Inc., Norfolk, VA, USA) with different background stiffness values were used (Fig. 1). The phantoms were rectangular (262 × 262 ×192 mm) with background stiffness values of 4 ± 1 and 16 ± 2 kPa to mimic a normal liver and a cirrhotic liver (LC), respectively. Background stiffness of phantoms was based on SWE measurement of Young's modulus in healthy patients over the range 4.5–5.5 kPa and cirrhosis cutoffs of 11 kPa in
Results
The mean and range of stiffness, CV and morphologic score values of inclusions with respect to influencing factors are outlined in Table 1.
Discussion
In this study, the stiffness measurement for inclusions using 2-D SWE was influenced by background stiffness and the depth at which the inclusion was located. Also, the lesion conspicuity of the inclusion body on elastography was influenced by its size. In the present study, the inclusions simulated the focal liver lesions that developed in the liver with various background stiffness values resulting from fibrosis and inflammation (Guibal et al., 2013, Park et al., 2015, Tian et al., 2016), and
Conclusions
Two-dimensional SWE assessment of small focal lesions in deep organs could be influenced by background stiffness and depth of focal lesions in quantitative stiffness estimation and by size in qualitative evaluation. To maintain the accuracy, reliability and spatial resolution of SWE results, there could be limitations to the depth and size of target lesions.
Acknowledgments
The study was funded by the research fund of Samsung Medison Inc. This work was also supported by the Soonchunhyang University Research Fund.
Declaration of Competing Interest
The authors declare no competing interests.
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