Design and Verification of Finite-element Simulation Modeling for Vibro-acoustic Effect

Ultrasound-stimulated vibro-acoustography (USVA), a speckle-free and high-resolution elastic imaging method, uses two focused ultrasound beams with slightly offset frequency to generate dynamic acoustic radiation force (ARF) which can drive the tissue to produce localized harmonic vibration. The low-frequency vibrated tissue radiates sound energy outward with its elastic properties. The complex energy conversion process can be also called vibro-acoustic effect (VAE), which is able to be divided into three different stages, generation of source sound field, low-frequency vibration caused by dynamic ARF and propagation of secondary sound field. However, there is currently no such complete finite-element modeling for the complex energy conversion process due to challenges of computational complexity caused by finite-element calculation and multi-physical field coupling. Based on the physical mechanism and theoretical derivation, a complete finite-element simulation modeling for VAE is designed in which the computational cost is able to be reduced effectively and acoustic-structure coupled interfaces are utilized to couple sound field and solid mechanical field together. To verify the feasibility of the proposed modeling for VAE, three stages of VAE are discussed respectively. For the availability of source sound field, the dynamic and steady state of high-frequency wave propagation as well as nonlinear interaction of two focused ultrasound beams in the focal area can be obtained and visualized by the proposed simulation modeling for VAE. Regarding to the verification of low-frequency vibration caused by dynamic ARF, the simulation result demonstrates that the frequency of vibration velocity at focus coincides with the offset frequency. Utilizing the propagation of secondary sound field in the proposed modeling for VAE, the relationship among pressure amplitude of secondary sound field at the observation point, offset frequency and vibration velocity is discussed to confirm that the relative elasticity of tissue in the focal area can be obtained. The proposed complete finite-element simulation modeling for VAE is expected to provide visual and thorough understanding and guidance of VAE for future research.