Original Contribution
2-D Shear Wave Elastography for Focal Lesions in Liver Phantoms: Effects of Background Stiffness, Depth and Size of Focal Lesions on Stiffness Measurement

https://doi.org/10.1016/j.ultrasmedbio.2019.08.006Get rights and content

Abstract

The aim of this study was to determine the factors influencing stiffness and conspicuity of focal lesions in deep organs by focusing on target properties using 2-D shear wave elastography (SWE). Two normal (4 ± 1 kPa) and cirrhotic (16 ± 2 kPa) liver-mimicking phantoms with spherical inclusions (23 ± 3 kPa) were used. Inclusions of three sizes (20, 15 and 10 mm in diameter) were arranged in a row at depths of 3, 5 and 7 cm. Two observers acquired quantitative stiffness values and a qualitative five-grade morphologic score at each inclusion using SWE. The coefficients of variation (CVs) of stiffness were calculated to assess measurement reliability. The generalized estimating equation was used to identify whether stiffness, CV and morphologic score were independent of background stiffness, depth and size of inclusions and observer. In the quantitative assessment, stiffness of the inclusion and CV were dependent on the type of phantom and depth of inclusion (p < 0.001). There were no significant differences in stiffness and CV according to the observer. Morphologic score differed significantly only in the size of the inclusion (p < 0.001). When the depth of the inclusion was 7 cm, the stiffness was the highest, and the 10 mm-sized inclusions had lower morphologic scores than the other inclusions (all p values < 0.001). In conclusion, 2-D SWE assessment of focal lesions could be affected by background stiffness and depth of focal lesions, and may be limited in evaluating focal hepatic lesions.

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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