Optimization Of High Intensity Focused Ultrasonic Transducer And Bio-heat Transfer In Tissues

High Intensity Focused Ultrasound(HIFU)is an emerging non-invasive technology for tumor treatment.Although HIFU has been widely used in the treatment of various benign and malignant solid tumors,there are still some key issues that need to be resolved.The major work involving HIFU optimization in this dissertation includes two aspects,i.e.,(1)Design and implementation of a HIFU metamaterial focusing transducer to enhance focusing and temperature elevation at the focus;(2)Prediction of temperature responses in biological tissue induced by HIFU was studied by using Fourier and non-Fourier bio-heat transfer models.Extraordinary acoustic transmission(EAT)at a specific frequency can be achieved via artificially structured acoustic material,and focusing enhancement of transducer is achieved.In this dissertation,a metamaterial focusing transducer was designed and fabricated via artificially acoustic structure to enhance the focusing efficiency of HIFU transducer.Enhanced acoustic focusing was demonstrated by both measuring and simulating sound pressure distribution of metamaterial focusing transducer and traditional concave transducer,as well as temperature elevation generated in the tissue.The results demonstrated that the metamaterial focusing transducer with sub-wavelength periodic structure exhibited stronger capacity in elevating sound pressure and temperature elevation in the focal region compared with the conventional concave transducer.In addition,the frequency of EAT close to Wood's anomaly could be modulated by spherically curved periodic slot arrays,which will be conducive to reveal the physical mechanism of EAT.The Pennes equation is widely used to describe the thermal effect in biological tissues.However,the heat conduction term of this equation is based on Fourier's law,which assume that the heat propagation is a transient process and still has its defects.In this dissertation,the thermal effect of biological tissue induced by HIFU was studied.By using mimicking tissue phantom and ex vivo bovine liver tissues.The prediction of temperature responses in biological tissue from Pennes equation,based on Fourier's law,and Thermal wave model of bio-heat transfer(TWMBT)and Dual phase-lag(DPL),based on non-Fourier's law,were compared.The results demonstrated that:(1)for the homogeneous tissue phantom,temperature elevation could be accurately estimated through Pennes at the initial stage of irradiation,while deviation appeared as sonication went on;(2)for the heterogeneous liver tissues,measured temperature responses were closer to the predictions of non-Fourier models,especially DPL;(3)accurate prediction of the temperature response in biological tissues using DPL could be achieved by first evaluating the phase-lag that characterizes microstructural thermal interaction.In conclusion,the research of this dissertation has a positive effect on the key issues of HIFU,which can further promote the wide application of HIFU in clinical treatment.