Sound And Temperature Distribution Induced By Focused Ultrasound In Two-layer Tissues And Its Physical Property Analysis

High intensity focused ultrasound (HIFU), as a non-invasive technology for tumor treatment, has been the focus in medical ultrasound research field. Not only for ultrasound hyperthermia treatments of tumors but also for determining the safety limits of diagnostic ultrasound equipment, it is of vital importance to calculate the ultrasound field distribution and temperature evaluation. The properties of transducer and the properties of biology tissues are two vital aspects that affect the sound field distribution. Generally speaking, the sound pressure fields have been calculated by the Rayleigh-Sommerfeld diffraction integral over the surface of the radiator for each field point of interest. To simplify and accelerate calculations, the biological tissues are assumed as homogeneous tissues, with same acoustic properties and thermal properties. And reflection and refraction at tissue interfaces have been ignored. However, these interfaces have become more important for high intensity ultrasound, which is sharply focused, for treating tumors. So refraction and reflection at tissues interfaces should be taken into consideration when calculating the pressure fields. The critical factor in treatment is the temperature measurement of the focal point and the effective treating region. Acoustic energy is absorbed and then transferred to heat energy in biological tissues. The distribution of acoustic energy is one of the determining factors of the heat distribution. Besides, the acoustic properties and thermal properties of biological tissues, such as thermal conductivity coefficient, heat capacity and so on, are other key factors. The difference of thermal properties between different tissues impacts directly temperature field distribution. Compared with other temperature measurement such as invasive technology and non-invasive technology, theoretical calculation can give more information to assist carrying out the treatment plan and help estimate the temperature distribution during the treatment as well. And it is helpful in analyzing the impact on temperature field imposed by acoustic properties and thermal properties of biological tissues.In this paper, the sound distribution in single-layer tissue is calculated by Rayleigh integral algorithm. On this basis, a two-layer biological tissue model has been established and the theoretical sound field produced by HIFU in it has been calculated. Under the model of single focus, the effects on the sound field distribution have been analyzed, which are brought by refraction and reflection induced by different impedance of two-layer tissue.Sound energy will be absorbed by biological tissue during its propagation. Focused ultrasound has been used as the thermal source in two-layer biological tissue. To simulate the temperature evaluation, ANSYS has been used for calculation. The thermal conductivity and heat capacity have been changed to analyses the effects on temperature distribution under conditions of different focal locations.Last but not least, the phased-array HIFU system and sound field measurement system with hydrophone have been put together to conduct experiments with in vitro biological tissues. Experiments have been used to explain the impact on the sound pressure at the focus brought by the tissue's thickness,acoustic attenuation and refraction and reflection occurred on the interface of different tissues. And the phased-array HIFU system and temperature measurement system with thermal probe have been put together to conduct experiments with in vitro two-layer biological tissue. Experiments have been used to explain the impacts on the temperature field brought by thermal properties.In this paper, the refraction and reflection of focused ultrasound at the interface of the two-layer biological tissue have been gropingly researched. The effects on sound field distribution and temperature distribution caused by focal position, acoustic properties and thermal properties have been studied. It is of great importance for the formulation and carrying out of the treatment plan and the analysis of the real temperature distribution during the treatment.