The Simulation Research On The Heating Biological Medium Of High Intensity Focused Ultrasound

As one of the most efficient cancer treatments recently, high intensity focused ultrasound(HIFU) tumor therapy uses convergent and penetrable ultrasonic waves to converge the low-energy of ultrasonic waves in vitro on the tumor targets in vivo. HIFU destroys the proteins in the tumor tissues by coagulation necrosis in a short period of time without damaging the neighbouring healthy tissues, thereby achieving a goal of minimal invasive or non-invasive treatment.The simulation research on the sound and heat distribution in HIFU treatment may help us to objectively analyze the clinical treatment effect and provide proper guidances for the clinical dose of ultrasonic waves. In this paper, according to the related theoretical models, we have conducted simulation researches on the heating of biological medium by HIFU. We have analyzed the distributions of sound intensity, temperature and thermal dose in biological tissues under the linear or nonlinear acoustic environment. We have also quantitatively identified factors that affect the distributions of sound intensity, temperature and thermal dose. The influences of blood vessels on the temperature rise, thermal dose changes and the heat distribution in the tumor tissues were also analyzed in the presence of blood vessels. We have done some works as follows:Firstly, we have divided the concave spherical surface into several infinitesimals and calculated the sound intensity in each infinitesimal. By Rayleigh integral, we have obtained the linear sound intensity distribution of the concave spherical transducer. We have also analyzed the acoustic field and the sound pressure of the nonlinear sound wave with SBE equation. The relationship between the sound intensity and the parameters of the sound source as well as the characteristics of tissues were further explored. The results of MATLAB simulation have demonstrated that the focusing effect of the nonlinear conditions is better than the linear conditions. The sound intensity within the focus increases with the increase of ultrasonic frequency and nonlinear coefficient, but decreases when the radius of curvature and the tissue attenuation coefficient increase.Secondly, on the basis of the linear and nonlinear sound intensity, the Pennes heat conduction equation was solved with finite difference method, then the dose distribution was calculated to determine the thermal damage degree of HIFU on tissues. The effects of the frequency of ultrasonic sound source, the radius of curvature, the tissue attenuation coefficient, the initial pressure, and the nonlinear coefficient on the thermal field in the focal region was discussed in detail. The simulation results revealed that higher frequency of ultrasonic sound and greater initial pressure lead to greater thermal damage in the focus, and more obvious thermal effect produced by the interactions between tissues; bigger radius of curvature and larger attenuation coefficient may reduce the focusing effect and more attenuation of ultrasonic energy and smaller thermal injury; the focusing effect of nonlinear ultrasound is better, so bigger nonlinear coefficient may cause more obvious thermal injury.Thirdly, Using vascular convection heat transfer model, we have analyzed the influence of the size of blood vessels, the deviation from focus, blood perfusion rate and blood flow velocity on the temperature rise and thermal dose in the tissues at the presence of blood vessels. The simulation results showed that the thermal damage area is divided into two parts, and the thermal damage area becomes smaller due to the existence of blood vessel; when the size of blood vessel, blood flow velocity or blood perfusion rate increase, the ranges of effective thermal dose obviously decrease; the closer blood vessels to the center of the focal region, the greater the effect.