Systematic quantification of differences in shear wave elastography estimates between linear-elastic and viscoelastic material assumptionsa).

Quantitative, accurate, and standardized metrics are important for reliable shear wave elastography (SWE)-based biomarkers. For over two decades, the linear-elastic material assumption has been employed in SWE modes. In recent years, viscoelasticity estimation methods have been adopted in a few clinical systems. The current study aims to systematically quantify differences in SWE estimates obtained using linear-elastic and viscoelastic material assumptions. An acousto-mechanical simulation framework of acoustic radiation force impulse-based SWE was created to elucidate the effect of material viscosity and shear modulus on SWE estimates. Shear modulus estimates exhibited errors up to 72% when a numerical viscoelastic phantom was assessed as linearly elastic. Shear modulus estimates of polyvinyl alcohol phantoms between rheometry and SWE following the Kelvin-Voigt viscoelastic model assumptions were not significantly different. However, the percentage difference in shear modulus estimates between rheometry and SWE using the linear-elastic assumption was 50.1%-62.1%. In ex vivo liver, the percentage difference in shear modulus estimates between linear-elastic and viscoelastic methods was 76.1%. These findings provide a direct and systematic quantification of the potential error introduced when viscoelastic tissues are imaged with SWE following the linear-elastic assumption. This work emphasizes the need to utilize viscoelasticity estimation methods for developing robust quantitative imaging biomarkers.

Viscoelastic Characterization of Phantoms for Ultrasound Elastography Created Using Low- and High-Viscosity Poly(vinyl alcohol) with Ethylene Glycol as the Cryoprotectant.

Ultrasound elastography enables noninvasive characterization of the tissue mechanical properties. Phantoms are widely used in ultrasound elastography for developing, testing, and validating imaging techniques. Creating phantoms with a range of viscoelastic properties relevant to human organs and pathological conditions remains an active area of research. Poly(vinyl alcohol) (PVA) cryogel phantoms offer a long shelf life, robustness, and convenient handling and storage. The goal of this study was to develop tunable phantoms using PVA with a clinically relevant range of viscoelastic properties. We combined low- and high-viscosity PVA to tune the viscoelastic properties of the phantom. Further, phantoms were created with an ethylene glycol-based cryoprotectant to determine whether it reduces the variability in the viscoelastic properties. Scanning electron microscopy (SEM) was performed to evaluate the differences in microstructure between phantoms. The density, longitudinal sound speed, and acoustic attenuation spectra (5-20 MHz) of the phantoms were measured. The phantoms were characterized using a shear wave viscoelastography approach assuming the Kelvin-Voigt model. Microstructural differences were revealed by SEM between phantoms with and without a cryoprotectant and with different PVA mixtures. The longitudinal sound speed and attenuation power-law fit exponent of the phantoms were within the clinical range (1510-1571 m/s and 1.23-1.38, respectively). The measured shear modulus (G) ranged from 3.3 to 17.7 kPa, and the viscosity (η) ranged from 2.6 to 7.3 Pa·s. The phantoms with the cryoprotectant were more homogeneous and had lower shear modulus and viscosity (G = 2.17 ± 0.2 kPa; η = 2.0 ± 0.05 Pa·s) than those without a cryoprotectant (G = 3.93 ± 0.7 kPa; η = 2.6 ± 0.14 Pa·s). Notably, phantoms with relatively constant viscosities and varying shear moduli were achieved by this method. These findings advance the development of well-characterized viscoelastic phantoms for use in elastography.

Characterizing Viscoelastic Polyvinyl Alcohol Phantoms for Ultrasound Elastography.

Ultrasound phantoms mimic the acoustic and mechanical properties of native tissues. Polyvinyl alcohol (PVA) phantoms are used extensively as models for validating ultrasound elastography approaches. However, the viscous properties of PVA phantoms have not been investigated adequately. Glycerol is a viscous liquid that has been reported to increase the speed of sound of phantoms. This study aims to assess the acoustic and viscoelastic properties of PVA phantoms and PVA mixed with glycerol at varying concentrations. The phantoms were fabricated with 10% w/v PVA in water with varying concentrations of glycerol (10%, 15% and 20% v/v) and 2% w/v silicon carbide particles as acoustic scatterers. The phantoms were subjected to either one, two, or three 24-h freeze-thaw cycles. The longitudinal sound speeds of all PVA phantoms were measured, and ranged from 1529 to 1660 m/s. Attenuation spectroscopy was performed in the range of 5 to 20 MHz. The measured attenuation followed a power-law relationship with frequency, wherein the power-law fit constants and exponents ranged from 0.02 to 0.1 dB/cm/MHzn and from 1.6 to 1.9, respectively. These results were in agreement with previous reports for soft tissues. Viscoelasticity of PVA phantoms was assessed using rheometry. The estimated values of shear modulus and viscosity using the Kelvin-Voigt and Kelvin-Voigt fractional derivative models were within the range of previously-reported tissue-mimicking phantoms and soft tissues. The number of freeze-thaw cycles were shown to alter the viscosity of PVA phantoms, even in the absence of glycerol. Scanning electron microscopy images of PVA phantoms without glycerol showed a porous hydrogel network, in contrast to those of PVA-glycerol phantoms with non-porous structure. Phantoms fabricated in this study possess tunable acoustic and viscoelastic properties within the range reported for healthy and diseased soft tissues. This study demonstrates that PVA phantoms can be manufactured with glycerol for applications in ultrasound elastography.