Study On Theoretical Issues Of Focusing Ultrasound For Heating Biological Medium

Much attention has been more and more paid to the biologic effects produced by ultrasound, especially focused ultrasound in medium. One of the applications is used to treat all kinds of tumors using heat produced by the focused ultrasound in biologic medium. Primary characteristic is its noninvasion, minimal invasion and controllability during ultrasound therapy. But many difficulties and problems are usually met during ultrasound treatment today, such as migration and deformation of the focal region and unpredictability of the damage to the tissue induced by cavitation effect, etc. They are can be usually observed in clinical and is also a nonlinearity problem during insolation in bio-medium; some problems, such as a longer time therapy and low sound synthesis, can also be usually met during treatment when treatment region is changed; In addition, temperature distribution in biological medium is changed because of acoustic nonlinearity, which causes overshoot and oscillation of temperature control during hyperthermiaThese issues stated above can be divided into two categories. One is the physical mechanism needs to be understood further, another is some control methods of sound field and temperature need to be improved. Based on the the problems mentioned previously, we did some work as follows:First, some acoustic phenomena produced by the interaction of ultrasound and medium are given a detailed analysis, especially the sound field distribution affected by the nonlinearity of the medium is studied in detail. The nature of the temperature elevation to the medium is discovered.Secondly, an accurate temperature state-space model is obtained by the Fourier transformation of Pennes equation. The model incudes not only conduction, convection and blood perfusion, but also nonlinearity of the bio-medium, and it provides a general model for temperature control of tumor treatment in different nonlinear sound field.Thirdly, a control model of one single-point temperature is presented by computing the sound field produced by a self-focused concave sphere and its temperature distribution change with time. The target temperature is controlled with a self-tuning regulator (STR) using the temperature state-space model and the power of ultrasound transducer is regulated by temperature feedback. Numerical simulation is performed in different treatment region and with different blood perfusion. The results show that there is on overshoot during temperature elevation and no oscillation after achieving the desired temperature. The control model of one-single temperature can simplify the hyperthermia control system, reduces the treatment cost and gives a guarantee for temperature control of target.Fourthly, based on pseudo-inverse algorithm, a direct weight formula of sound pressure by using a method of amplitude and phase to the controlled sound pressure is presented and is used to preset the values to the controlled points, and thus the amplitude and phase of each element can be obtained by solving the pseudo-inverse matrix. Some expressions of the previous pseudo-inverse algorithm are simplified and a few definitions are given renew, which makes acoustic focusing to be simpler and more flexible, and enhances the sound synthetic efficiency.Fifthly, cavitation is phenomenon in focusing ultrasound field and has its occurrence probability, and it bring about a great impact on the temperature of the focal region. It is important to understand the occurrence of the acoustic cavitation. The expansion ratio is used to predict the occurrence probability of inertial cavitation (ICOP) by solving the modified Rayleigh-Plesset equation for nonlinear bubble dynamics in viscoelastic media. The simulation results are in qualitatively good agreement with the experiments. The influence on the temperature distribution of the focal region are demonstrated by investigations of the some kinds of cavitation bubble and cavitation effects during tumor therapy, and some analysis are given to these phenomena.Sixthly, based on the characteristics of high temperature and pressure, a conduction-radiation bubble dynamics model is presented. The temperature inside the bubble under the condition of conduction-radiation is higher than that of the adiabat at the same condition during the compression of bubble. The temperature difference becomes lager with reducing the thickness of the bubble. The temperature inside the bubble under the condition of conduction-radiation is relatively stable. Temperature in the focal region is computated with time with the time-average cavitation effect. the results show that the temerature of the focal region can generate a jump as the incident wave reaches a certain sound pressure.