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Numerical and experimental investigation of scattering of focused ultrasonic waves at solid-fluid interfaces
Focused ultrasound is an emerging therapeutic modality that has found clinical use for the treatment of a number of serious medical conditions. It is an attractive treatment modality due to its non-ionizing and noninvasive nature. Active research is currently taking place worldwide, both to improve existing applications and to discover and develop novel applications of the technique. For example, high-intensity focused ultrasound (HIFU) can be used for the ablation of tissue, such as in the case of prostate cancer. However, targeting the tissue deeper inside the body remains challenging due to the increased attenuation and scattering of the ultrasonic waves. Ultrasonic transmission and reflection under such conditions have scarcely been the focus of previous research. In this thesis, the partial and complete obstruction of the ultrasonic beam from a HIFU transducer at bone phantoms is numerically investigated. Pressure waveforms are modeled in water and in a tissue-mimicking bone phantom for a focused transducer with center frequency of 670 KHz, aperture of 58mm and focal length of 73.19 mm. A 1MPa sinusoidal turn burst pressure load is applied to induce ultrasonic waves. This thesis contains three numerical approaches to simulate the propagation of focused ultrasonic waves. First, acoustic-solid transient simulations are conducted for various bone obstruction configurations. It is shown that the presence of a bone phantom has a significant effect on the scattering of the waves as well as the maximum pressure amplitude at the focal point. Next, piezoacoustic simulations are conducted with different piezoelectric material thicknesses to optimize the pressure amplitude that is applied in acoustic simulations. It is shown that the thickness of the piezoelectric material has a significant effect on the maximum pressure amplitude at the focal point. Furthermore, temperature simulations are conducted to evaluate the thermal effect of focused ultrasonic waves at the focal region. These results show that the temperature rise is dependent on factors such as excitation frequency and duration of the treatment. Moreover, a custom-made positioning system is designed for controlling the movement of the transducer in experimental measurements. In addition, a diffraction-based shadowgraph technique is introduced for the ultrasound visualization in laboratory experiments. Finally, the simulation models for a single focused transducer is expanded to multiple transducers. The main advantages of performing numerical simulations, in comparison with experimental measurements, is that each physical parameter affecting the results can be independently and accurately controlled. Overall, it is shown that a combination of measurements and modeling is necessary to enable accurate characterization of HIFU fields. Thus, this thesis provides a reference based on numerical and experimental setups for HIFU therapy.