The Effect Of Microbubbles On High Intensity Focused Ultrasound Focal Region

In recent years,noninvasive or minimally invasive high intensity focused ultrasound(High Intensity Focused Ultrasound,HIFU)tumor therapy has attracted the attention of many researchers.The treatment method is to make a low energy ultrasonic emited out of human body focusing to the tumor in the human body,a therapy by tissue absorb ultrasonic energy and make the temperature reached above 60 degrees lead to the tumor tissue coagulative necrosis in a short period of time.While curing deep tumors in human body,due to acoustic propagation distance and the required transmission path may be through the strong attenuation of bone,lead to attenuation of ultrasonic energy which may result in the shortage of energy of focus and can not make the tumor tissue coagulative necrosis,meanwhile,the therapeutic focal region of single acoustic is relatively small,for large volume tumor therapy with multiple superimposed focal region,the required treatment need a long time and may cause complications.In order to effectively and safely treat deep body tumor and large volume tumor,the focal energy shortage and long treatment time is an urgent problem need to be solved for the clinical treatment of HIFU,microbubbles which has synergistic effect in the treatment of HIFU attracted the attention of many researchers.ObjectiveIn order to effectively and safely treat deep body tumor and large volume tumor,solve the problem of the focal energy shortage and long treatment time,Many researchers have proposed the use of microbubble contrast agent to improve the therapeutic effect of HIFU.In this paper,a tissue mimicking numerical simulation model is been established,the distribution of the focal region under different conditions is been discussed by using numerical simulation method,by analyzing the transducer input power,excitation frequency,phantom parameters and microbubble initial distribution conditions on the formation of HIFU focal region,and verified by the tissue mimicing phantom with egg white and microbubble contrast agent as the experimental object,in order to provide reference data for the application of microbubble contrast agent in the clinical treatment of HIFU.MethodsIn this study,the numerical simulation model is established based on the structure parameters of the concave spherical self focusing ultrasonic transducer.The pressure field and temperature was simulated based on the acoustic propagation equation,Yang-Church bubble motion equation,bio-heat transfer equation,the finite difference time domain method and Runge-Kutta method,and tested by experiments of tissue mimicing phantom with SonoVue microbubbles,to study the effect on the formation of HIFU focal region.ResultsAs the initial gas volume fraction in the phantom increasing,the volume of focal region above 60 ? increasing and focal region moves close to the transducer direction;when the gas volume fraction is 2.5×10-6,to form a focal region in the phantom surface result in failing to form effective focal region.As the input power increasing,the volume of focal region above 60? increasing,the peak temperature of the focal region increasing gradually and the focal position is almost unchanged.When the input power is 30 W,the focal region size of numerical simulation and phantom experiment is almost same,when the power is 40W~60W the focal region volume of numerical simulation is smaller than of phantom experimental.As working frequency of transducer increasing,the volume of focal region above 60? increasing,the peak temperature of the focal region increasing and focal region moves away from the transducer direction,the long axis and the short axis of the focal region increase with the increase of the working frequency,and the slope becomes smaller.As working frequency of transducer increasing,the volume of focal region above 60? increasing,the peak temperature of the focal region increasing and focal region moves away from the transducer direction,the long axis and the short axis of the focal region increase with the increase of the working frequency,and the slope becomes smaller.As the viscosity of the phantom increasing,the volume of focal region above 60? decreasing,the peak temperature of the focal region decreasing and focal region moves away from the transducer direction,the long axis and the short axis of the focal region decrease with the increase of the viscosity.As shear modulus of the phantom increasing,the volume of focal region above 60? first increasing and then decreasing,the peak temperature and the peak pressure of the focal region first increasing and then decreasing and the position of focal region is almost unchanged,the long axis and the short axis of the focal region first increasing and then decreasing with the increase of the viscosity.As the initial bubble radius of the phantom increasing,the volume of focal region above 60? increasing,the peak temperature of the focal region increasing and focal region moves away from the transducer direction,the long axis and the short axis of the focal region increase with the increase of the working frequency,and the slope becomes smaller.ConclusionsMicrobubbles can increase the volume of therapeutic focal region above 60? of HIFU;Increasing the input power,initial bubble radius and enhance the working frequency can increase the volume of focal region,as the input power increasing,the shape of focal region may change;Increase working frequency and the initial bubble radius,the focal region move away from the transducer direction;As the viscosity of the phantom increasing,peak temperature of focal region is lower,the volume of focal region is smaller,the focal region moves away from the direction of the transducer;With the increase of the shear modulus of the phantom,the volume of focal region increased first and then decreased,and t the focal region moves away from the direction of the transducer.