Modelling The Nonlinear Acoustic Field Of HIFU Transducer And Comparative Study Of Scanning Strategy For Tissue Lesion Generation

High Intensity Focused Ultrasound (HIFU) is a noninvasive treatment that delivers high intensity ultrasound energy into the targeted area of tissue within the body without damage to intervening tissues. As a new method for cancer treatment, it becomes one of hot spots during past several decades. The major work of this dissertation includes two aspects, i.e. modeling the nonlinear acoustic field of an acoustic lens transducer, and comparing the temperature and lesion distributions generated by two scanning strategies.Acoustic lens is usually used to focus acoustic energy. In this work, a modeling approach to compute the nonlinear acoustic field generated by a flat piston transmitter with an attached aluminum lens is proposed. In this approach, the geometrical parameters (radius and focal length) of the virtual source are initially determined by Snell's Refraction Law and then adjusted based on the Rayleigh integral result in the linear case. Then this virtual source is used with the nonlinear spheroidal beam equation (SBE) model to predict the nonlinear acoustic field in the focal region. The calculated nonlinear result is compared with those from the Westervelt and KZK equations for a focal intensity of7kW/cm2Results indicate that this approach could accurately describe the nonlinear acoustic field in the focal region with less computation time.When a lesion with large volume is to be targeted, the HIFU energy may be delivered through two strategies, i.e. sequential discrete mode and continuous scanning mode. Lesion formation and temperature distribution induced are numerically and experimentally investigated via two energy delivering strategies. Measurements were performed on tissue-mimicking phantom using a1.12-MHz single-element focused transducer at an acoustic power of75W. Simulations are presented based on the combination of KZK equation and bio-heat equation. The results show that, in the sequential discrete mode, obvious saw-tooth-like contours could be observed for the peak temperature distribution and the lesion boundaries with the increasing interval space between two adjacent exposure points. In the continuous scanning mode, more uniform peak temperature distributions and lesion boundaries would be produced, and the peak temperature values decreases significantly with the increasing scanning speed. In addition, compared to the sequential discrete mode, the continuous scanning mode could achieve higher treatment efficiency (lesion area generated per second) with a lower peak temperature. The present studies suggest that the peak temperature and tissue lesion resulted from the HIFU exposure could be controlled by adjusting the transducer scanning speed, which is important for improving the HIFU treatment efficiency.In addition, some other contents are also presented in this thesis, including:an approach combined acoustic field, temperature distribution and bubbles is proposed to model the effects of boiling bubble on acoustic and temperature distribution, and the nonlinear propagation in multi-layer biological tissues for strong focused ultrasound based on SBE model.In conclusion, the works on the nonlinear acoustic field from HIFU transmitter and lesion formation in different scanning strategies will be helpful for the further application of HIFU in clinical ultrasonic therapy.