A Nonlinear Acoustic Field Model Study Based On The Abdominal Wall

Medical ultrasound imaging is widely applied clinically with the advantages of simplicity, safety, non-invasive detection and low cost. Acoustic parameters vary in different types of biological tissue, due to which complicated diffraction, scattering and nonlinear phenomenon will occur during the propagation of ultrasound in heterogeneous tissue, resulting in geometry and energy distribution distortion. Therefore, it is significant to investigate the propagation of ultrasound in biological tissue which is also the foundation of the focused beam distortion correction algorithm. With the constraints of current acoustic detection technology, it is difficult to obtain the experimental data of pressure and energy distribution of the acoustic beam in biological tissue. Thus the simulation of ultrasound propagation is still an effective method to get the distribution data of the medical ultrasound field.This thesis proposed a numerical method of stagger scheme finite difference to solve the coupled full-wave equation for nonlinear ultrasound using OpenCV toolkit based on the digital human abdominal wall image against the defects of current medical ultrasound simulation. Then the distribution of ultrasound field in human abdominal wall tissue was simulated and the characteristics were studied. Besides, with the SONIX TOUCH color ultrasonic diagnosis system and the acoustic measuring system, the distribution data were experimentally measured in the medium of water and both the experimental and simulation results were compared in order to verify the validity of the numerical method proposed in this thesis. At last the application of the numerical method in tissue harmonic imaging is investigated.In this thesis the human abdominal wall tissue was as the medium of the ultrasound propagation instead of random medium, which reflected the complexity of the biological tissue more practically. With the calculated model proposed, it is easier to obtain a converged solution compared to the traditional Finite Difference Time Domain (FDTD) method. Compared with homogeneous medium, when sound speed in tissue is uniform and density unchanged, the acoustic energy decreases only1.8dB in the focal region; when density in tissue is uniform and sound speed unchanged, the energy decreases3.8dB in the focal region, which is almost the same as heterogeneous tissue. Thus, the primary factor of the aberration of focused beam is heterogeneous distribution of the tissue sound speed. The instantaneous pressure and energy data were obtained, which could be reference to evaluating biological safety of ultrasound doses, developing commercial simulation toolkit of nonlinear ultrasound and accelerating the product manufacture using nonlinear sound field theory.