Research On Cavitation Effect Enhanced High Intensity Focused Ultrasound Therapy

As a new method of tumor treatment,High-intensity focused ultrasound(HIFU)attracts much attention due to its noninvasive and targeted characteristics.HIFU technology can focus the ultrasound energy into the target area,inducing the rapid temperature rise in the target tissue,and forming the irreversible thermal coagulation of the target tumor tissue,thereby kill the tumor cells without damaging the surrounding normal tissues and achieving noninvasive ultrasound tumor treatment.Cavitation effect refers to the action of ultrasound on the process of the formation,vibration and collapse of micro-bubbles in the liquid,and the effect of this complex kinetic process on the biological tissue.Thermal effect is the main mechanism of HIFU treatment of cancer.There is a high sound intensity in the focal area of the HIFU sound field,and the focal negative sound pressure amplitude can reach about 100 MPa,which is high enough to cause the ultrasonic cavitation effect.Although some earlier studies suggested that cavitation should be avoided as much as possible during HIFU treatment,with the increasingly mature regulation and control technology of the cavitation effect,more and more attention has been paid to the application of cavitation effect in the targeted drug delivery and HIFU treatment in recent years.As an effective method to gather energy,the cavitation effect will significantly enhance the thermal effect of HIFU when cavitational microbubbles exist in the focal field.However,the cavitation effect is characterized by non-controllability and unpredictability,so the quantitative monitoring of cavitation effect has been a research difficulty.In this thesis,we investigate the enhancement of cavitation effect in the HIFU treatment.The main contents include two aspects:(1)Real-time video monitoring of acoustic cavitation in HIFU therapy based on commercial B-ultrasound imaging system;(2)Studying the enhancement of cavitation effect on thermal damage and temperature rise in HIFU treatment theoretically and experimentally.Precise real-time monitoring of cavitation effects is a prerequisite for effective use of cavitation to enhance HIFU therapy in HIFU treatment.In this thesis,the experimental device was designed and constructed for the real-time monitoring of cavitation effect based on a commercial B-imaging system.Using two-dimensional digital image processing algorithm to eliminate interference fringes generated by HIFU in B-mode images,quantitatively analyzing the area changes of B ultrasound imaging in the bright area,discussing and analyzing the ultrasonic cavitation caused by different HIFU energy in the gel imitation,then the real-time monitoring of the cavitation effect in HIFU treatment can be achieved.In order to study the effect of cavitation on the temperature rise and tissue damage in HIFU treatment,gel and liver tissue in vitro,were chosen as the research object in this thesis.Firstly,using the nonlinear acoustic KZK equation and the bio-heat transfer Pennes equation,the temperature rise and tissue lesion caused by HIFU under different acoustic parameters were calculated.Then using B-imaging system and temperature measurement system for real-time monitoring of temperature rise and tissue lesion.By comparing the theoretical and experimental values of temperature rise and tissue lesion,the role of pulse length and sound pressure amplitude in cavitation effect can be analyzed and discussed.Finally,the enhancement of the cavitation effect in the tissue lesion formation and the temperature rise in HIFU treatment process was discussed.In this thesis,the effect of cavitation on enhancement of temperature rise and tissue lesion during HIFU treatment process was studied theoretically and experimentally.The related research results will further promote the clinical application of HIFU.