The Research Of Ultrasonic Quantitative Measurement Methods For Biological Tissue Viscoelasticity

Ultrasound elastography is a kind of noninvasive technique for detecting body lesions using elasticity information. It excites the tissue to deform or vibrate by certain method, and detects the deformation or vibration using different imaging modalities, thus estimates Young’s modulus or shear modulus of the tissue.Among various kinds of ultrasound elastography proposed by the researchers, Shearwave Dispersion Ultrasound Vibrometry(SDUV) features quantitative assessment of both elasticity and viscosity. It uses acoustic radiation force to excite the region of interest of the tissue, causing shear waves in the local area, measuring phase velocities of shear wave with ultrasonic pulse-echo technique, thus obtains the tissue elasticity and viscosity simultaneously. Biological tissue are inherently viscoelastic, therefore, the mechanical properties characterized by the SDUV should agree with the nature of biological tissue.However, some key problems about the SDUV technique have not yet been solved, mainly including:(1) The accuracy of viscoelasticity of the SDUV has not been validated.(2) Is the viscosity of the tissue indispensable for the elasticity measurement?(3) Is the SDUV effective for the grading of liver fibrosis?(4) Which is the optimal mechanical model suitable to describe the mechanical properties of liver fibrosis?This paper carried a series of study in order to solve the above problems. Firstly, the varied viscoelasticity of the phantoms were realized by increasing the concentration of castor oil or lard oil. Secondly, the SDUV and Acoustic Radiation Force Impulse(ARFI) were combined for analyzing the viscoelasticity of the castor oil phantoms. The results revealed that the viscosity is indispensable for the tissue elasticity measurement and ignoring the viscosity will be biased estimate of tissue elasticity. Thirdly, the viscoelasticity measurements of the SDUV and Dynamic Mechanical Analysis(DMA) were compared, this paper confirmed that the SDUV elasticity measurements are consistent with the DMA elasticity measurements but the accuracy of the SDUV viscosity measurements depend on the frequency range of shear waves. Fourthly, a comprehensive evaluation was conducted on the viscoelasticity measurements of the SDUV and the DMA for different rat liver fibrosis stages. This paper confirmed that the SDUV is effective for the grad ing of liver fibrosis. Finally, three different mechanical models were used for describing mechanical properties of rat liver fibrosis. The results showed that the Zener model is optimal in terms of fitting errors while it has the same effectiveness in rat liver fibrosis grading with the Voigt model.To summarize, the original contributions of the thesis are: fabricating the oil- in- gelatin phantoms with varied viscoelasticity, revealing that the viscosity is an indispensable measurement for the tissue elasticity assessment, validating the accuracy of the SDUV elasticity measurement but its viscosity depending on the frequency range of shear waves, confirming the effectiveness of the SDUV for the grading of rat liver fibrosis, and revealing that the Zener model is the optimal mechanical model while there is no significant difference between the Zener and Voigt models in their performance on liver fibrosis staging.