Study On The Thermal And Cavitation Effects Of Therapeutic Ultrasound In Biological Medium

Ultrasound therapy is a commonly used form of physical therapy.There are many types of therapeutic ultrasound.As a typical therapeutic ultrasound,high intensity focused ultrasound?HIFU?has the advantages of noninvasive and non-ionization,and it is an ideal surgical method for the treatment of common tumors.Ultrasound propagation in biological medium will produce thermal and cavitation effects,and these effects have an important influence on the therapeutic effect of ultrasound,and they are also the key issue in the field of ultrasound therapy that need further study.In this paper,the thermal and cavitation effects of ultrasound in biological medium have been studied in three aspects:?1?Temperature monitoring based on ultrasonic nonlinear harmonic was studied.?2?The prediction of thermal lesion caused by ultrasound in biological medium was studied.?3?Cavitation effect of multi-frequency ultrasound in biological medium was studied.The specific research work is as follows:Firstly,by analyzing the influence of the change of focal temperature on the amplitude of nonlinear harmonic,one method of temperature monitoring based on the nonlinear harmonic was proposed.A fiber-optic hydrophone was used to passively receive the scattering echo signal generated by HIFU irradiation in biological medium in vitro.At the same time,a thermocouple was used to measure focal temperature change in real time.The correlation between the amplitude of nonlinear harmonic and the temperature was studied.The results showed that the nonlinear harmonic was related to the temperature change,and temperature had influence on harmonic amplitude.The harmonic amplitude increased first and then decreased with the temperature rise,and the harmonic amplitude changed with the temperature was related to the nonlinear and absorption effects.Moreover,the ratio of P₂/P₁,P₃/P₁ and P₃/P₂ increased first and then decreased with the increase of temperature,and the ratio of P₂/P₁,P₃/P₁ and P₃/P₂ decreased faster in porcine muscle than in porcine liver.Secondly,according to the characteristics of the traditional method of HIFU treatment,which used the characteristic parameters of constant temperature?room temperature or body temperature?to predict thermal lesion,the influence of characteristic parameters of different temperature on the prediction of thermal lesion was studied.A coupled acoustic-thermal model based on KZK nonlinear acoustic equation and Pennes biological heat transfer equation was proposed,and the dynamic characteristic parameters were used to predict thermal lesion,and the relationship between harmonic amplitude ratio and thermal lesion was analyzed.Based on the known experimental data of the characteristic parameters of biological medium and the method of data fitting,the relationship between the characteristic parameters and the temperature was obtained.Through the simulation and calculation of only changing the acoustic parameters,only changing the thermal parameters,and changing the acoustic and thermal parameters at the same time,the difference and change rule of the influence of acoustic and thermal parameters on the prediction of thermal lesion of medium were revealed.The simulation results showed that the focal temperature and thermal lesion predicted by different acoustic parameters,acoustic and thermal parameters decreased first and then increased with the increase of temperature corresponding to the parameters,while the focal temperature and thermal lesion predicted by different thermal parameters increased first and then decreased with the increase of the temperature corresponding to the parameters.Compared with the acoustic parameters,acoustic and thermal parameters of different temperature and the prediction of focal temperature and thermal lesion by thermal parameters,the prediction result was too high or too low for traditional method.The difference between the acoustic parameters and acoustic and thermal parameters that led to the thermal denaturation increased with the increase of temperature,and the thermal lesion caused by continuous heating was larger than that caused by segment heating.Moreover,the thermal lesion predicted by dynamic characteristic parameters was larger than that predicted by traditional method,and the thermal lesion increased with the decrease of harmonic amplitude ratio.Thirdly,the radius change and collapse time of bubbles in four different mediums of water,blood,brain and liver and under the combined radiation of multi-frequency ultrasound was simulated by using the Yang-Church cavitation model.The effects of the combination mode of multi-frequency ultrasound,the peak negative pressure and its duration,the phase angle difference and the polytropic index on the transient cavitation threshold in different medium were studied,which provided a reference for optimizing the combination of multi-frequency ultrasound to reduce the transient cavitation threshold.The simulation results showed that when the single frequency ultrasound and the highest frequency of multi-frequency combined ultrasound was the same,the bubble expansion of multi-frequency combined ultrasound was larger than that of single frequency ultrasound,but when the single frequency and the lowest frequency of multi-frequency combined ultrasound were the same,the bubble expansion of multi-frequency combined ultrasound was smaller than that of single frequency ultrasound.The larger the bubble radius of R_max/R_minin in the same medium was,the longer the bubble collapse time was,and the increase of medium viscosity reduced the intensity of bubble growth and collapse.Compared with the combination of three frequencies with lower frequency,the frequency difference had more obvious effect on the transient cavitation threshold of the combination of three frequencies with higher frequency.The greater the peak negative pressure of ultrasonic signal or the longer the duration of peak negative pressure was,the lower the corresponding transient cavitation threshold was.Moreover,when the phase angle difference of multi-frequency excitation was zero,the corresponding transient cavitation threshold was the lowest,while the change of polytropic index had little effect on the transient cavitation threshold of multi-frequency combination.In this paper,the research on the thermal effect and cavitation effect of therapeutic ultrasound can further promote the clinical application of therapeutic ultrasound.