8C-2 Error Estimates in Shear Wave Speed and Tissue Material Properties in Shear Wave Dispersion Ultrasound Vibrometry

Shear wave speed measurements are used in elasticity imaging to find the shear elasticity and viscosity of tissue. A technique called shear wave dispersion ultrasound vibrometry (SDUV) has been introduced to use the dispersive nature of shear wave speed to locally estimate the material properties of tissue. Shear waves are created using a multifrequency ultrasound radiation force, and the propagating shear waves are measured a few millimeters away from the excitation point. The shear wave speed is measured using a repetitive pulse-echo method and Kalman filtering to find the phase of the harmonic shear wave at two different locations. Using the following relationship, c_s = omega_sDeltar/Deltaphi where omega_s is the shear wave frequency, Deltar is the distance between measurement points, Deltaphi is the phase difference, the shear wave speed, c_s, can be estimated. A viscoelastic Voigt model and the shear wave speed measurements at different frequencies are used to find the shear elasticity (mu₁) and viscosity (mu₂) of the tissue. The purpose of this paper is to assess the accuracy of the SDUV method over a range of different values of mu₁ and mu₂. A motion detection model of a vibrating scattering medium was used to analyze measurement errors of vibration phase in a scattering medium. To assess the accuracy of the SDUV method, we modeled the propagation of phase errors into errors in the shear wave speed and material property estimates while varying parameters such as shear stiffness and viscosity, shear wave amplitude, Deltar, signal-to-noise ratio (SNR) of the ultrasound pulse-echo method, and the frequency range of the measurements. We performed an experiment in a section of porcine muscle to evaluate variation of the aforementioned parameters on the shear wave speed and material property measurements and to validate the computer model. The model showed that errors in the shear - wave speed and material property estimates were minimized by maximizing shear wave amplitude, pulse-echo SNR, Deltar, and the frequency range used. The experimental model showed optimum performance could be obtained for Deltar = 3-6 mm, SNR ges 20 dB, with a frequency range is 100-600 Hz, and with a shear wave amplitude on the order of a few microns down to 0.5 mum. We present a computational model and experimental approach to analyze errors in measurements of shear wave speed and material properties. The model provides a basis to explore different parameters related to implementation of the SDUV method. The experiment confirmed conclusions made by the model, and the results can be used for optimization of SDUV.