Study Of The Nonlinear Acoustic Field And Tissue Lesion Generated By A Spherical Cavity Transducer

Spherical cavity transducer is a special focused ultrasound transducer, which could break through the limit of the dimension of focal region and have potential applications in high intensity focused ultrasound (HIFU) therapy. This thesis study the nonlinear acoustic field and tissue lesions created by the spherical cavity transducer. Research works includes:(1) a numerical model is developed for describing the nonlinear acoustic field generated from the spherical cavity transducer using the finite difference time domain (FDTD) method to solve the nonlinear Westervelt equation, and results are compared with those obtained from the conventional curved transducer with the same configuration.(2) tissue lesion induced by a spherical cavity transducer is investigated based on nonlinear acoustics theory and bio-heat transfer theory. The numerical simulations are compared with the experimental measurements, and the optimization of the design of the spherical cavity transducer is also discussed.(3) For tissue lesion creation, two therapy modes are compared, ie. sequential discrete mode and continuous scanning mode, and the applied acoustic power in the continuous mode is also discussed.The results demonstrate:(1) For a spherical cavity transducer with diameter120mm working at0.6MHz, results are compared with those obtained from the conventional curved transducer with the same configuration. Results indicate that as the exciting acoustic pressure is100kPa, the spherical cavity transducer could increase focusing gain by about8.5times and reduce-6dB focal region by20times in z direction.(2)A430kHz spherical cavity transducer with a diameter of240mm is used to generate tissue lesion for liver tissues. The result shows that tissue lesion with less than a wavelength level can be formed with2s HIFU exposures at a source acoustic pressure of53kPa. This works demonstrate that the spherical cavity transducer can be used in accurate HIFU tumor treatment.(3) The continuous scanning mode can achieve tissue lesions with more uniform boundary, which could be used in the HIFU therapy with spherical cavity transducer.