Carolina Amador

Shearwave dispersion ultrasound vibrometry (sduv) on swine kidney

Shearwave dispersion ultrasound vibrometry (SDUV) is used to quantify both tissue shear elasticity and shear viscosity by evaluating dispersion of shear wave propagation speed over a certain bandwidth (50 to 500 Hz). The motivation for developing elasticity imaging techniques is the desire to diagnose disease processes. However, it is important to study the mechanical properties of healthy tissues; such data can enhance clinical knowledge and improve understanding of the mechanical properties of tissue. The purpose of this study is to evaluate the feasibility of using SDUV for in vitro measurements of renal cortex shear elasticity and shear viscosity in healthy swine kidneys. Eight excised kidneys from female pigs were used in these in vitro experiments and a battery of tests was performed to gain insight into the material proper ties of the renal cortex. In these 8 kidneys, the overall renal cortex elasticity and viscosity were 1.81 ± 0.17 kPa and 1.48 ± 0.49 Pa-s, respectively. In an analysis of the material properties over time after excision, there was not a statistically significant difference in shear elasticity over a 24-h period, but a statistically significant difference in shear viscosity was found. Homogeneity of the renal cortex was examined and it was found that shear elasticity and shear viscosity were statistically different within a kidney, suggesting global tissue inhomogeneity. In creases of more than 30% in shear elasticity and shear viscosity were observed after immersion in 10% formaldehyde. Finally, it was found that the renal cortex is rather anisotropic. Two values for shear elasticity and shear viscosity were measured depending on shear wave propagation direction. These various tests elucidated different aspects of the material properties and the structure of the ex vivo renal cortex.

Shear Elastic Modulus Estimation From Indentation and SDUV on Gelatin Phantoms

Tissue mechanical properties such as elasticity are linked to tissue pathology state. Several groups have proposed shear wave propagation speed to quantify tissue mechanical properties. It is well known that biological tissues are viscoelastic materials; therefore, velocity dispersion resulting from material viscoelasticity is expected. A method called shearwave dispersion ultrasound vibrometry (SDUV) can be used to quantify tissue viscoelasticity by measuring dispersion of shear wave propagation speed. However, there is not a gold standard method for validation. In this study, we present an independent validation method of shear elastic modulus estimation by SDUV in three gelatin phantoms of differing stiffness. In addition, the indentation measurements are compared to estimates of elasticity derived from shear wave group velocities. The shear elastic moduli from indentation were 1.16, 3.40, and 5.6 kPa for a 7%, 10%, and 15% gelatin phantom, respectively. SDUV measurements were 1.61, 3.57, and 5.37 kPa for the gelatin phantoms, respectively. Shear elastic moduli derived from shear wave group velocities were 1.78, 5.2, and 7.18 kPa for the gelatin phantoms, respectively. The shear elastic modulus estimated from the SDUV, matched the elastic modulus measured by indentation. On the other hand, shear elastic modulus estimated by group velocity did not agree with indentation test estimations. These results suggest that shear elastic modulus estimation by group velocity will be bias when the medium being investigated is dispersive. Therefore, a rheological model should be used in order to estimate mechanical properties of viscoelastic materials.

Loss tangent and complex modulus estimated by acoustic radiation force creep and shear wave dispersion

Elasticity imaging methods have been used to study tissue mechanical properties and have demonstrated that tissue elasticity changes with disease state. In current shear wave elasticity imaging methods typically only shear wave speed is measured and rheological models, e.g. Kelvin-Voigt, Maxwell and Standard Linear Solid, are used to solve for tissue mechanical properties such as the shear viscoelastic complex modulus. This paper presents a method to quantify viscoelastic material properties in a model-independent way by estimating the complex shear elastic modulus over a wide frequency range using time-dependent creep response induced by acoustic radiation force. This radiation force induced creep method uses a conversion formula that is the analytic solution of a constitutive equation. The proposed method in combination with shearwave dispersion ultrasound vibrometry is used to measure the complex modulus so that knowledge of the applied radiation force magnitude is not necessary. The conversion formula is shown to be sensitive to sampling frequency and the first reliable measure in time according to numerical simulations using the Kelvin-Voigt model creep strain and compliance. Representative model-free shear complex moduli from homogeneous tissue mimicking phantoms and one excised swine kidney were obtained. This work proposes a novel model-free ultrasound-based elasticity method that does not require a rheological model with associated fitting requirements.

In vitro renal cortex elasticity and viscosity measurements with shearwave dispersion ultrasound vibrometry (SDUV) on swine kidney

Shearwave dispersion ultrasound vibrometry (SDUV) is used to quantify both tissue shear elasticity and shear viscosity by evaluating dispersion of shear wave propagation speed over a certain bandwidth (50 to 500 Hz). The motivation for developing elasticity imaging techniques is the desire to diagnose disease processes. However, it is important to study the mechanical properties of healthy tissues; such data can enhance clinical knowledge and improve understanding of the mechanical properties of tissue. The purpose of this study is to evaluate the feasibility of using SDUV for in vitro measurements of renal cortex shear elasticity and shear viscosity in healthy swine kidneys. Eight excised kidneys from female pigs were used in these in vitro experiments and a battery of tests was performed to gain insight into the material proper ties of the renal cortex. In these 8 kidneys, the overall renal cortex elasticity and viscosity were 1.81 ± 0.17 kPa and 1.48 ± 0.49 Pa-s, respectively. In an analysis of the material properties over time after excision, there was not a statistically significant difference in shear elasticity over a 24-h period, but a statistically significant difference in shear viscosity was found. Homogeneity of the renal cortex was examined and it was found that shear elasticity and shear viscosity were statistically different within a kidney, suggesting global tissue inhomogeneity. In creases of more than 30% in shear elasticity and shear viscosity were observed after immersion in 10% formaldehyde. Finally, it was found that the renal cortex is rather anisotropic. Two values for shear elasticity and shear viscosity were measured depending on shear wave propagation direction. These various tests elucidated different aspects of the material properties and the structure of the ex vivo renal cortex.

Measurements of swine renal cortex shear elasticity and viscosity with Shearwave Dispersion Ultrasound Vibrometry (SDUV)

Fibrosis has been associated with altered tissue structure affecting the biomechanical properties of the organs, quantifiable as elasticity and viscosity. Renal fibrosis threatens kidney viability. Evaluation of renal fibrosis by conventional imaging modalities is difficult. Importantly, early detection of renal fibrosis may guide therapy and eliminate invasive biopsy procedures. A newly emerging method called Shearwave Dispersion Ultrasound Vibrometry (SDUV) offers a potential tool to determine renal elasticity and viscosity in vivo. SDUV quantifies both elasticity and viscosity by evaluating dispersion of shear wave propagation speed versus its frequency. The purpose of this study was to evaluate the feasibility of SDUV for in vitro measurements of renal cortex elasticity and viscosity in swine kidney.

Improvement of Shear Wave Motion Detection Using Harmonic Imaging in Healthy Human Liver.

Quantification of liver elasticity is a major application of shear wave elasticity imaging (SWEI) to non-invasive assessment of liver fibrosis stages. SWEI measurements can be highly affected by ultrasound image quality. Ultrasound harmonic imaging has exhibited a significant improvement in ultrasound image quality as well as for SWEI measurements. This was previously illustrated in cardiac SWEI. The purpose of this study was to evaluate liver shear wave particle displacement detection and shear wave velocity (SWV) measurements with fundamental and filter-based harmonic ultrasound imaging. In a cohort of 17 patients with no history of liver disease, a 2.9-fold increase in maximum shear wave displacement, a 0.11 m/s decrease in the overall interquartile range and median SWV and a 17.6% increase in the success rate of SWV measurements were obtained when filter-based harmonic imaging was used instead of fundamental imaging.

Viscoelastic measurements on perfused and non-perfused swine renal cortex in vivo

Renal fibrosis threatens kidney viability and fibrosis has been associated with altered tissue structure affecting the biomechanical properties of the kidney, such as elasticity and viscosity. Although there are limited studies regarding viscoelastic properties of renal tissue, an in vivo renal MRE study suggests that shear elasticity changes as a result of hemodynamic variables. A newly emerging method called Shearwave Dispersion Ultrasound Vibrometry (SDUV) offers a tool to determine renal elasticity and viscosity in vivo. SDUV quantifies both elasticity and viscosity by evaluating dispersion of shear wave propagation speed versus its frequency. The purpose of this study is to evaluate the feasibility of SDUV for in vivo measurements of elasticity and viscosity on normal swine kidney consequent to acute changes in renal hemodynamics.

Effects of phase aberration on acoustic radiation force-based shear wave generation

Tissue elasticity is measured by shear wave elasticity imaging methods using acoustic radiation force to create the shear waves. Reliable tissue elasticity measurements are achieved with strong shear waves. Phase aberration and tissue attenuation can hamper the generation of shear waves for in vivo applications. In this study we explored how phase aberration affects ultrasound focusing for creating shear waves and evaluate the resulting shear wave amplitude and the shear wave velocity. A Verasonics ultrasound system equipped with a linear, a curved linear, and a phased array transducer (L7-4, C4-2 and P4-2) was used. An excised piece of swine belly tissue consisted of the skin, subcutaneous fat, and muscle were placed on top of the elastic phantom to evaluate the shear wave produced when transmitting through the different layers. The ultrasound frequency used for three transducers was varied to evaluate the resulting shear waves with the different tissue layers. Analysis of shear wave production with real tissue layers showed that large fat layers and combinations of skin, fat, and muscle defocused the ultrasound beam most, thereby decreasing the shear wave amplitudes.

Viscoelastic tissue mimicking phantom validation study with shear wave elasticity imaging and viscoelastic spectroscopy

Acoustic radiation force-based shear wave elasticity (SWE) methods are used to characterize soft tissue pathologies by quantifying tissue viscoelastic (VE) properties. To adequately evaluate SWE methods measurements of VE properties, it is important to characterize VE properties of tissue mimicking phantoms with an independent method. In this study a VE tissue mimicking phantom was developed and its concentration was modified to vary the shear viscosity. Three VE phantoms were studied with SWE and hyper frequency viscoelastic spectroscopy (HFVS). The VE properties were quantified by fitting a Kelvin-Voigt fractional derivative model to the measured shear wave speed. The mean shear elasticity of the three phantoms was from 3.4 to 3.7 kPa, the mean shear viscosity was from 1.7 to 17.4 Pas and the power of the fractional derivative model varied from 0.6 to 0.9.

Evaluation of frequency characteristics of shear waves produced by unfocused and focused beams

Measurements of viscoelasticity with shear wave velocity dispersion requires measurements over a large bandwidth. In this study, we explored the parameters that modulate the frequency characteristics of shear waves induced using radiation force push beams. We used a Verasonics ultrasound scanner equipped with a linear array transducer. We performed measurements of shear wave motion induced using both focused and unfocused ultrasound beams. Measurements were made in elastic phantoms with shear moduli of 1, 4, and 16 kPa. The number of elements used for the unfocused beams were varied from 8 to 24, and for the focused beams from 16 to 128. The shear wave motion was tracked using plane wave imaging, and a one-dimensional autocorrelation algorithm applied to the acquired in-phase/quadrature data. At each pixel we calculated the fast Fourier transform of the data and found the center frequency, center-of-gravity, and −3 dB bandwidth. We compared the frequency characteristics from the different push beams. The ...