Viscosity Plane-Wave UltraSound (Vi PLUS) in the Evaluation of Thyroid Gland in Healthy Volunteers-A Preliminary Study

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.

Assessment of Thyroid Stiffness and Viscosity in Autoimmune Thyroiditis Using Novel Ultrasound-Based Techniques

The estimation of viscosity by measuring the shear-wave dispersion (SWD) using ultrasound 2D shear-wave elastography (SWE) is becoming more and more popular. Recent research suggests that SWD can be used in addition to 2D-SWE (shear-wave speed) to diagnose diffuse liver disease. Viscosity was studied for the assessment of normal thyroid tissue. This study aims to evaluate the use of viscosity measurements in patients with chronic autoimmune thyroiditis using the SuperSonic MACH^®30 ultrasound machine (Hologic SuperSonic Imagine, Aix-en-Provence, France) which provides the Vi PLUS mode for viscosity and the 2D SWE PLUS mode for shear-wave speed measurements. Valid measurements were obtained in 308 cases, 153 with chronic autoimmune thyroiditis (CAT) and 155 with no thyroid pathology (95.95% feasibility of the methods). The differences between the healthy group and the CAT group were statistically significant both for Vi PLUS (2.5 ± 0.4 vs. 2.8 ± 0.5, p < 0.0001) and for 2D-SWE PLUS (13.5 ± 3.3 vs. 23.1 ± 8.3, p < 0.0001). The diagnostic performance was poor for Vi PLUS alone (AUC = 0.69; cut-off > 2.5 Pa·s, se = 68.6%; sp = 64.52%) and good for 2D-SWE PLUS alone (AUC = 0.861; cut-off > 18.4 kPa, se = 69.9%; sp = 92.2%). Vi PLUS correlated with 2D-SWE PLUS, with the presence of CAT, the thyroid volume, levothyroxine replacement therapy and age. Statistically significant differences were found between the CAT subgroup receiving thyroid replacement therapy and the subgroup without therapy: 24.74 ± 8.33 vs. 21.93 ± 8.12 kPa for 2D-SWE (p = 0.0380) and 3 ± 0.5 vs. 2.7 ± 0.4 Pa·s for Vi PLUS (p = 0.0193). Elastography-based methods improve the classic ultrasound evaluation: 2D-SWE PLUS performed somewhat better in distinguishing CAT from normal thyroid tissue, while Vi PLUS made a slightly better assessment regarding the functional status.