Study Of The Ultrasound-induced Temperature Field In Biological Tissues Using Finite Element Method

In order to ensure the efficiency of ultrasound hyperthermia and High Intensity Of Focused Ultrasound (HIFU), it is needed to control the temperature of biological tissues, therefore the temperature measurement in the treatment is necessary. Thermocouple is often used in clinical treatment. Although it is a high accurate technique, but is needed to insert into the organ, this gives the patient pain, and therefore is not widely used. The non-destructive temperature measurement methods have been proposed, such as electrical impedance tomography, X-CT, MRI imaging, ultrasound, microwave, hot temperature compensation method and computer simulation, in which a computer simulation of temperature field is based on the Pennes bio-heat equation, considering the blood flow, the ultrasonic source strength and the measured temperature outside the skin. Because the biological tissues are multi-layer structures and their shapes are irregular, therefore the numerical method must be used to obtain the temperature field distribution.In this paper, firstly, Khokhlov-Zabolotaskaya-Kuznetsov (KZK) equation is used to get the sound field distribution of multi-layer tissues, a finite element software is used to calculate the Pennes equation, and axial-temperature distribution for multi-layer tissues is obtained under different conditions. The experimental results are compared with theoretical prediction and fit the theoretical results well. The results show that when the thickness of samples increase, more energy is absorbed by the anterior part of tissue compared with before, so the focus will get less energy, and the maximum temperature elevation will decrease, and the position of maximum temperature will also move backward. When the sound pressure at the surface of the transducer increases, the maximum temperature elevation of the sample will also increase, but the position of maximum temperature remains the same, therefore, the sound pressure at the surface of the transducer has a great influence on the temperature field. When the samples moves backward, less energy will be absorbed by the anterior part of tissue, so the focus will get more energy, and the maximum temperature elevation will increase, and the position of maximum temperature will also move forward. For two-layer tissue (fat-liver) and three-layer tissue (fat-muscle-liver), a part of energy is absorbed by fat sample and muscle sample, less energy will be absorbed at the focus, and the maximum temperature elevation will decrease, and the position of maximum temperature will also move backward. Our research show that the thickness and tissue parameters are very important for the hyperthermia, which must be taken into consideration during the hyperthermia with ultrasound.Based on the calculation of the Finite Difference method, COMSOL finite element software is used to simulate the coupling sound and temperature field and analyze the results under a variety of biological models, such as the different thickness, position, shape, irregularity of the tissue samples and the presence of ribs and skins, which are the important factors on the temperature field. The results of this study will provide useful information on ultrasound hyperthermia and HIFU.