Tissue Lesion Formation Generated By HIFU Transducer With Sub-wavelength Metamaterials

High Intensity Focused Ultrasound(HIFU)can concentrate sound energy to the focal area,and treat tumors based on ultrasound biological effects.Due to acoustic diffraction,sidelobe appear in the focal area.Excessive sidelobe intensity may cause damage to healthy tissue in the vicinity of the focal area.Increasing the main-side lobe ratio has become the design goal of HIFU transducers.In recent years,studies have shown that acoustic collimation effect and extraordinary acoustic transmission(EAT)can suppress acoustic diffraction and improve acoustic wave transmittance.In the design of HIFU transducers,the acoustic collimation effect of the acoustic metamaterial can be further used to enhance the focusing effect of the transducer and suppress sidelobe.In this paper,both theoretical calculations and experiments are used to study the tissue damage generated by HIFU focused transducers with sub-wavelength acoustic metamaterial.The metamaterial transducer used in this paper is fabricated by coupling the subwavelength periodic surface groove structure with the traditional self-focused transducer.Firstly,based on Helmholtz equation,Pennes biological heat transfer equation and thermal dose formula,the finite element method is used to establish an acoustic-thermal model and damage model to predict the biological tissue temperature field and coagulative necrosis formation during the radiation of metamaterial HIFU transducer.Subsequently,an ex vivo bovine liver tissue experiment is carried out to study the formation and morphology of tissue damage,and the result is also compared to that caused by the conventional focused transducer.Based on the results from previous paper,metamaterial transducers can deposit more acoustic power than conventional ones under same excitation situation,therefore in the experiment,acoustic energy is used as a variable,the damage area and temperature rise of the two transducers under the same acoustic energy are recorded to study the controlling effect of the metamaterial transducer.The research results show that: Compared with conventional transducers,the sidelobe suppression effect of the metamaterial transducer is better.As the focal area under the same sound power is declined,the damage volume is smaller,therefore the treatment safety and accuracy are improved.In addition,this paper also proposes a method for optimizing the metamaterial focused transducer with a hole in centre based on genetic algorithm.The results show that the design goal of suppressing sidelobe is achieved,and the accuracy of calculation and the total time save are taken into account.The research work in this thesis can bring positive significance for expanding the application of metamaterial focusing transducers in HIFU treatment,and further promoting the ultrasound therapy application of acoustic metamaterial in the future.