Aiping Yao

Shear Wave Dispersion Ultrasound Vibrometry Based on a Different Mechanical Model for Soft Tissue Characterization

Ultrasound vibrometry can measure the propagation velocity of shear waves in soft tissue noninvasively, and the shear moduli of tissue can be estimated inversely from the velocities at multiple frequencies. It is possible to choose the appropriate model for tissue characterization from mathematical methods and analysis of model behaviors. The three classic models, Voigt, Maxwell, and Zener, were applied to fit the velocity measurements and estimate shear moduli of porcine livers with different thermal damage levels and different storage times. The Zener model always provided the best estimation of the moduli with the minimum errors in our experiments. Unlike the Voigt and Maxwell models, the moduli of the Zener model cannot be used to indicate damage levels in porcine livers directly, but the creep and relaxation behaviors of the Zener model are effective.

Quantification of liver stiffness and viscosity with SDUV: In vivo animal study

Measurement of liver elasticity (i.e., stiffness) can be used as a noninvasive alternative to liver biopsy to stage liver fibrosis, a condition afflicting hundreds of millions of patients worldwide. Quantitative measurement of stiffness (in unit of Pascal) is required because liver fibrosis is a diffuse disease where abnormality is not confined to a local region and there is no normal background tissue to provide contrast. Shearwave Dispersion Ultrasound Vibrometry (SDUV) uses shear wave propagation speed measured in tissue at multiple frequencies (typically in the range of hundreds of Hertz) to solve quantitatively for both tissue elasticity and viscosity. A shear wave is stimulated within the tissue by an ultrasound push beam and monitored by a separate ultrasound detect beam. The phase difference of the shear wave between two locations along its propagation path is used to calculate shear wave speed within the tissue. An intermittent pulse sequence is developed to facilitate one array transducer for both push and detect function. Feasibility of this pulse sequence is demonstrated by SDUV measurements in porcine liver using both a dual transducer prototype and a modified commercial ultrasound scanner.

6G-4 Measurement of Shear Wave Using Ultrasound and Kalman Filter with Large Background Motion for Cardiovascular Studies

Phases of tissue harmonic motions, induced by the ultrasound radiation force, over a distance can be detected by pulse echo ultrasound and Kalman filter. However, large background motions due to cardiovascular motion distort the weak harmonic vibration. The background motion can be modeled by the Wiener process or random-walk process. The harmonic motion and the background motion are represented by a fourth-order differential equation and four state variables. Based on this model, the Kalman filter estimates phase shifts of shear wave over a short distance in a tissue region from large background motions. The study shows that the estimates are valid when the velocity and acceleration of background motion are less than 3 m/s and 3 m/s/s, respectively

Shear Wave Propagation in Soft Tissue and Ultrasound Vibrometry

Studies have found that shear moduli, having the dynamic range of several orders of magnitude for various biological tissues [1], are highly correlated with the pathological statues of human tissue such as livers [2, 3]. The shear moduli can be investigated by measuring the attenuation and velocity of the shear wave propagation in a tissue region. Many efforts have been made to measure shear wave propagations induced by different types of force, which include the motion force of human organs, external applied force [4], and ultrasound radiation force [5].

Rapid shear wave measurement for SDUV with broadband excitation pulses and non-uniform sampling

We propose a method using broadband tone burst excitation pulses with non-uniform sampling for Shearwave Dispersion Ultrasound Vibrometry (SDUV). The method allows long excitation pulses with a high PRF of detection pulses to induce a strong tissue motion and obtain strong detection signal. The method also simultaneously estimates all harmonics of tissue motion induced by the broadband excitation pulses so that the shear wave can be rapidly estimated with minimized bias and distortion.

Impacts of the capsule on estimation of shear viscoelasticity of livers

The impact of the collagenous capsule of livers on the estimation of shear wave speeds in liver tissues is studied. Shear wave is induced using the method of Shear Dispersion Ultrasound Vibrometry (SDUV) and Orthogonal Frequency Ultrasound Vibrometry (OFUV). Shear wave speeds from 100 Hz to 800 Hz are measured at 0.4 mm and 4.9 mm below the capsule of swine livers in vitro. The study finds that the shear speeds of superficial tissues are significantly different from deep tissues. The Voigt, Maxwell, and Zener models are applied to fit the speed measurements and solve for shear moduli. The Zener model provides robust estimates of the moduli with the minimum estimation errors in different tissue conditions. While the Voigt model requires higher frequencies for stable and accurate estimates of shear moduli of livers, the Maxwell and Zener model provides stable estimates with a limited frequency range, which is valuable for SDUV in vivo applications.

Ultrasound vibrometry using orthogonal- frequency-based vibration pulses

New vibration pulses are developed for shear wave generation in a tissue region with preferred spectral distributions for ultrasound vibrometry applications. The primary objective of this work is to increase the frequency range of detectable harmonics of the shear wave. The secondary objective is to reduce the required peak intensity of transmitted pulses that induce the vibrations and shear waves. Unlike the periodic binary vibration pulses, the new vibration pulses have multiple pulses in one fundamental period of the vibration. The pulses are generated from an orthogonal-frequency wave composed of several sinusoidal signals, the amplitudes of which increase with frequency to compensate for higher loss at higher frequency in tissues. The new method has been evaluated by studying the shear wave propagation in in vitro chicken and swine liver. The experimental results show that the new vibration pulses significantly increase tissue vibration with a reduced peak ultrasound intensity, compared with the binary vibration pulses.

Orthogonal Frequency Ultrasound Vibrometry

New vibration pulses are proposed to increase the power of shear waves induced by ultrasound radiation force in a tissue region with a preferred spectral distribution. The new pulses are sparsely sampled from an orthogonal frequency wave composed of several sinusoidal signals. Those sinusoidal signals have different frequencies and are orthogonal to each other. The phase and amplitude of each sinusoidal signal are adjusted to control the shape of the orthogonal frequency wave. Amplitude of the sinusoidal signal is increased as its frequency increases to compensate for higher loss at higher frequency in the tissue region. The new vibration pulses and detection pulses can be interleaved for array transducer applications. The experimental results show that the new vibration pulses significantly increases induced tissue vibration with the same peak ultrasound intensity, compared with the binary vibration pulses.Copyright

Reverberation reduction in vibro-acoustography using channel estimation method

Vibro-acoustography (VA) displays the vibroacoustic response of the object (tissue) induced by two intersecting ultrasound beams of different frequencies. VA image of a given object may vary depending on boundary conditions. Such variations are attributed to the reverberation of the sound emanating from the object in the acoustic environment of the experiment. This effect can be explained in terms of acoustic propagation through multiple paths before reaching the receiving hydrophone. The goal of this study is to reduce VA image variations caused by the acoustic multipath. Two methods based on channel estimation are developed to evaluate the effect of multipath and correct the received acoustic data from the object. The first method uses two ultrasound excitation pulses having different length for every image location. Another method uses tissue responses of different excitation amplitudes to remove reverberation and measure tissue nonlinearity.

Two-dimensional spectrum estimation for flow with multiple velocities using maximum entropy

We have developed a high-resolution method to estimate multiple velocities of flows within one sample volume of pulse echo ultrasound for small flows. The method uses maximum entropy method (MEM) to estimate the covariance matrix of a two-dimensional (2D) signal when data length is limited. The estimated covariance is used to calculate high-resolution 2D power spectrum of flows. The simulation and experiment results show that our method provides high-resolution estimations for multiple velocities when data length is limited and small flows are located.