剪切波对HIFU经颅聚焦形成温度场影响的数值仿真研究

在高强度聚焦超声经颅治疗时,既有纵波又有剪切波,为了保障该治疗方法的安全有效性,有必要分析剪切波对HIFU治疗温度场的影响。该文基于人体头颅CT数据和曲率半径为150 mm的256阵元的半球相控换能器建立三维高强度聚焦超声经颅声波传播模型,利用时域有限差分法结合Westervelt声波非线性传播方程、动量方程、质量守恒方程和Pennes生物热传导方程数值仿真其形成温度场,研究在相同输入功率、不同聚焦角度条件下对应阵元数进行激励时,剪切波对换能器形成温度场的影响。结果表明,随换能器聚焦角度减小,在几何焦点处形成的焦域面积逐渐增大,考虑剪切波形成的温度场达到65?C所需时间逐渐延长,焦点前移程度越大;在相同聚焦角度条件下,考虑剪切波的温度场达到65?C所需时间更短,旁瓣更少,在颅骨处的温度更高,对焦点前移几乎没有影响;随换能器聚焦角度减小,考虑剪切波的模型形成的焦域面积变化范围更大;幂指数函数形式对不同聚焦角度下焦域面积大小的拟合优度高,可预测不同聚焦角度换能器形成的焦域面积。

Numerical simulation of the effect of shear wave on the temperature field of HIFU transcranial focusing

Ultrasound includes long wave and shear wave. In order to ensure the safety and effectiveness of the treatment, it is necessary to analyze the effect of shear wave on field area of High intensity focused ultrasound (HIFU) treatment. In this paper, a three-dimensional HIFU transcranial acoustic wave propagation model is established based on human skull CT data and a 256-array hemispherical phase-controlled transducer with a radius of curvature of 150 mm. The Westervelt nonlinear acoustic propagation equation and momentum equation, as well as mass conservation equation and Pennes bioheat equation are used to calculate the temperature field by Finite Difference Time Domain (FDTD).The effect of shear waves on the transducer temperature field is investigated when the corresponding input elements are excited at different focus angles. The results show that, as the focus angle of the transducer decreases, the area of the focal region formed at the geometric focus gradually increases, as well as the time required for the temperature field formed by the shear wave to reach 65 °C is gradually extended, and the focus advance. Under the same focus angle condition, the time required for the shear wave temperature field to reach 65 °C is shorter, the side lobes are less, the temperature at the skull is higher, and there is almost no influence on the focus advance. As the focus angle of the transducer decreases, the range of the focal area formed by the model considering the shear wave is larger. The power exponential function has good fit in the focal area at different focus angles, and can predict the focal region formed by other focus angle transducer.