Measurement And Numerical Simulation In High Intensity Focused Ultrasound Transducer

In recent years, High Intensity Focused Ultrasound (HIFU) has re-emerged as a minimally invasive and non-ionizing therapeutic modality for the treatment of a variety of solid tumors, including both malignant and benign tumors. But for HIFU tumor therapy system, several potential complications such as skin burns and incomplete coagulation of tissue damage may occur, and it can not be guarantee the safety of this treatment. Hence, it is necessary to forecast and measure effectively the acoustic field and temperature field of HIFU transducer.ObjectiveBy HIFU ultrasonic transducer for acoustic field in water distribution, isolated organizations and tissue phantom experiment of HIFU temperature field study, analysis and discussion on sound field distribution, temperature field in the focal region of HIFU and injury characteristics change. Thus supply for HIFU treatment planning with valid data and simulation methods.Research methods(1) Measuring sound pressure distribution in HIFU transducer by PT probe type hydrophone and method with the relative numerical simulation model results, verify and change the simulation method.(2) Irradiation on temperature membrane and tissue phantom by different conditions, discussed focal field shape variation and temperature distribution of focal region.(3) Irradiation on liver tissue in vitro in different conditions, discussing damage regional variation of size and shape.Result1. Measurement and Numerical simulation in HIFU transducer's acoustic field(1) When the excitation frequency was kept constant, the pressure at focal point got larger with the incentive power increasing, while the change in the focal length was negligible.(2) When the excitation frequency was kept constant, The focal point moved towards the transducer with the frequency increasing until the maximum sound pressure was gained at the resonant frequency (RF).(3) The distribution of acoustic pressure was almost the same both in the numeric simulation and experiment measure.(4) When the excitation frequency was lower than the resonant frequency, the same acoustic pressure as formed in the resonant frequency could be achieved by increasing the incentive power.2. Experiment in tissue phantom(1) In studying the distribution of temperature field in tissue phantom, the region which temperature between 68.5℃and 73.5℃is small and the region could be formed to ellipse shape once the temperature is higher than 73.5℃(2) After irradiation the tissue phantom by HIFU, the area which temperature between 68.5℃and 73.5℃is small and the area which temperature above 73.5℃is larger. When the incentive power was increased, the focal region with temperature higher than 73.5℃got larger, the long axis and short axis became longer, while the change in long axis was obvious than that in short axis.(3) After irradiation the tissue phantom by HIFU, the area which temperature between 68.5℃and 73.5℃is small and the area which temperature above 73.5℃is larger. When the exposure time was increased, the focal region with temperature higher than 73.5℃got larger, the long axis and short axis became longer, while the change in long axis was obvious than that in short axis.3. Experiment in vitro pig liver(1) When the incentive power was increased, the area of maximum focal region was increased and the length of long axis got larger a lot. The focal region of temperature field was an ellipse shape when the incentive power was 170w or 190w. Once the incentive power was increased to 220w, the focal region was irregular.(2) When the exposure time got larger, the area of maximum focal region was increased and the length of long axis got larger a lot. The focal region of temperature field was an ellipse shape when the exposure time was 8s or 10s. Once the exposure time was increased to 12s, the focal region was irregular. Conclusion(1) The pressure at focal point got larger with the incentive power increasing, while the change in the focal length was negligible; The focal point moved towards the transducer with the frequency increasing until the maximum sound pressure was gained at the resonant frequency (RF); The distribution of acoustic pressure was almost the same both in the numeric simulation and experiment measure.(2) With increased incentive power and extended exposure time, focal field becomes large, short and long axes are larger, long-axis range is greater than the shorter axis change rate in tissue phantom and liver tissue in vivo; under high excitation power and longer exposure time, pig liver injury in vitro region shapes may become irregular.