Numerical Simulation Of Tissue Acoustic Properties Effect On HIFU Temperature Field

As one of new cancer treatment methods,High Intensity Focused Ultrasound(HIFU)is a non-invasive or minimally invasive therapy and has been applied in clinical treatment.But as the testing and monitoring of temperature distribution during HIFU treatment have not been resolved,the safety of this therapy can not be ensured.ObjectiveNumerical simulate the affection of tissue acoustic properties on HIFU temperature field based on the measured acoustic parameters at different temperatures,in order to provide theoretical data and evidence for the reliability and security of HIFU therapy.MethodsIn this study,with the approximation of Westervelt Formula for the nonlinear propagation of ultrasound,proposed by Nagayoshi Moritato,and the Pennes Heat Transfer Equation,we simulate the effect of tissue acoustic properties on a HIFU temperature field using the FDTD method.First,the Mur first-order boundary conditions are used to calculate boundaries in a cylindrical coordinate system.Second,the calculation process is improved based on the acoustic propagation characteristics of HIFU in biological tissue,and the value of sound filed and temperature filed obtained by the improved algorithm is done and verified by error analysis.Then,based on the data of sound velocity and the attenuation coefficient at different temperatures measured by Bamber and Wende Shou,the variation of sound velocity and attenuation coefficient,and the distribution of acoustic and temperature fled in tissue during HIFU therapy is simulated using the FDTD method.Moreover,the effect of the two variable acoustic parameters and fat thickness on the therapeutic region is discussed in this paper.Results1.Boundary treatment and simulation method(1)Mur first-order absorbing boundary condition is used on the cylindrical computational domain boundary,and is verified to be able to meet the requirement of the HIFU numerical simulation.(2)The simulation algorithm is improved according to the HIFU propagation characteristics in tissue,besides,sound pressure stabilizing time,updating time and time step of temperature field is added in improved algorithm.Sound pressure average relative error and temperature average relative error of the new algorithm decrease,with the decrease of sound pressure updating time and time step of temperature field,or the increase of sound pressure stabilizing time.The sound pressure average relative error decreases with the passage of the exposure time.2.Effect of pig liver tissue acoustic properties on the HIFU temperature field(1)With the passage of the exposure time,the temperatureof the liver tissue rises,sound speed decreases and the attenuation coefficient increases in the focal region.(2)In the model of pig liver tissue,when considering the characteristics of sound velocity variation in liver tissue,the focus location moves away from the transducer with the extension of irradiation time.When the sound velocity varied,the focus temperature is 0.4? higher,the focal point moves 0.1mm away from the transducer,and therapeutic region is 0.1mm longer.(3)When considering the characteristics of the sound attenuation coefficient variation in liver tissue,the focal temperature is about 0.2? lower,and there are negligible changes in the therapeutic region and the position of focal point.3.Effect of bovine tissue acoustic properties on the HIFU temperature field(1)With the passage of the exposure time,tissue temperature rises,and sound speed decreases in both bovine fat and liver tissue.(2)In the model of the single pig liver tissue,when considering sound velocity variation characteristics,the focus location moves away from transducer with the extension of irradiation time.Compared with the results of constant sound velocity in tissue,the focal temperature is 1.27? higher,and the focal point moves 0.1mm away from the transducer when the focus point temperature is 90 ? after 7s of exposure.(3)In the double tissue model of fat and liver,when considering sound velocity variation characteristics,the focus location moves closer towards transducer with the extension of irradiation time.Compared with the results of constant sound velocity,the focal temperature is 2.28 ? higher,the focal point and focal region moved 0.3mm and 0.2mm closer to transducer respectively,and the therapeutic region is 0.2mm shorter when the sound velocity varied.(4)In double tissue model of fat and liver,when the fat thickness increased,the input sound intensity must be increased to obtain the same temperature in focal point,and the focal point and focal region moved away from transducer overall.Conclusions1.Mur first-order absorbing boundary conditions can solve the non-physical reflection problem on the FDTD calculation boundary of HIFU simulation in three-dimensional cylindrical coordinates.2.Improve theing calculation procedure based on the property that a relatively steady acoustic field forms in a certain period during HIFU treatment,and choosing key parameters appropriately could significantly improve computational efficiency without lowering the calculation accuracy.3.The characteristics that the attenuation coefficient vitiates with temperature nearly has no affect on HIFU focus region,while the variation of sound velocity with temperature could affect the focal region size,focus location and focal region location.The effect of this velocity variety property in fat tissue has more effect than that in liver tissue.