Development of Oil-in-Gelatin Phantoms for Viscoelasticity Measurement in Ultrasound Shear Wave Elastography

Abstract

Because tissues consist of solid and fluid materials, their mechanical properties should be characterized in terms of both elasticity and viscosity. Although the elastic properties of tissue-mimicking phantoms have been extensively studied and well characterized in commercially available phantoms, their viscous properties have not been fully investigated. In this article, a set of 14 tissue-mimicking phantoms with different concentrations of gelatin and castor oil were fabricated and characterized in terms of acoustic and viscoelastic properties. The results indicate that adding castor oil to gelatin phantoms decreases shear modulus, but increases shear wave dispersion. For 3% gelatin phantoms containing 0%, 10%, 20% and 40% oil, the measured shear moduli are 2.01 ± 0.26, 1.68 ± 0.25, 1.10 ± 0.22 and 0.88 ± 0.17 kPa, and the Voigt-model coupled shear viscosities are 0.60 ± 0.11, 0.89 ± 0.07, 1.05 ± 0.11 and 1.06 ± 0.13 Pa·s, respectively. The results also confirm that increasing the gelatin concentration increases shear modulus. For phantoms containing 3%, 4%, 5%, 6% and 7% gelatin, the measured shear moduli are 2.01 ± 0.26, 3.10 ± 0.34, 4.18 ± 0.84, 8.05 ± 1.00 and 10.24 ± 1.80 kPa at 0% oil and 1.10 ± 0.22, 1.97 ± 0.20, 3.13 ± 0.63, 4.60 ± 0.60 and 8.43 ± 1.39 kPa at 20% oil, respectively. The phantom recipe developed in this study can be used in validating ultrasound shear wave elastography techniques for soft tissues.

Introduction

Elastography, a tissue stiffness measurement technique, has been used for non-invasive diagnosis in many tumors of the breasts, liver, thyroid and prostate. Among different modalities, ultrasound-based elastography stands out because of the advantages of real-time imaging, relatively low cost and absence of ionizing radiation. With the rapid advance of elastography in the last two decades, tissue elastic properties have become relevant in establishing the existence of and quantifying the severity of diseases such as cancer and systemic sclerosis (Foucher et al., 2006, Huwart et al., 2006, Madhok et al., 2013, Muller et al., 2009).

Tissue-mimicking phantoms are essential to all medical imaging modalities because they are easily accessible and convenient to handle. Among phantom materials such as polymer gels, silicone gels and agar, gelatin stands out because of its mechanical properties, which are close to those of soft tissue (Zell et al. 2007). There are a number of commercially available tissue-mimicking phantoms including those from ATS Labs (Bridgeport, CT, USA), CIRS (Norfolk, VA, USA) and Gammex-RMI (Middleton, WI, USA). The phantoms provided by these companies have been well characterized in terms of sound speed, attenuation coefficient and Young's modulus, which is an elasticity modulus. Because biologic tissues contain a mixture of solid and fluid materials, they should be described in terms of both elasticity and viscosity (Sarvazyan et al. 1995). However, as far as the authors are aware, currently no viscous phantoms are commercially available. Although researchers have investigated the stability and non-linear elastic behavior of oil-in-gelatin phantoms, the viscous properties of these phantoms have not been fully investigated (Madsen et al., 2005, Madsen et al., 2006a, Pavan et al., 2010, Pavan et al., 2012). A recent study reported on the use of tissue viscous properties in determining histologic stages of liver steatosis (Barry et al. 2012). As viscosity may become a valuable metric in addition to elasticity in disease diagnosis and classification, we believe that there is a need for the assessment of phantom viscosity in conjunction with elasticity.

In this study, phantoms containing different concentrations of gelatin and castor oil were developed to mimic the elastic and viscous components of soft tissues. Sound speed and attenuation coefficient, along with the shear elasticity and viscosity of these phantoms, were measured and compared with results from previous studies. This article is constructed as follows: The Background section introduces the theoretical foundations for elastography with an emphasis on shear-wave ultrasound elastography. The Methods section presents (i) the procedures for fabrication of viscoelastic tissue-mimicking phantoms and (ii) methods for quantification of the acoustic and viscoelastic properties of these phantoms. Subsequently, the Results and Discussion summarizes our experimental findings. Finally, the Conclusions are stated.