Establishment Of Focused Ultrasound Clinical Treatment Dose Prediction System:Optimization Of Sound Field And Heat Transfer Model

Background:High intensity focused ultrasound（HIFU）is an emerging non-invasive treatment technology.The principle is to focus the in vitro low-energy ultrasound beam into the body to form a high-energy focus with a certain size through a certain focusing method.With the help of thermal effect,cavitation effect and mechanical effect,the focal tissue temperature is instantaneously increased to above 65°C,and the focus produces irreversible coagulative necrosis without damaging the surrounding normal tissue.At present,HIFU has been applied to the clinical treatment of various benign and malignant solid tumors such as breast cancer,liver cancer,pancreatic cancer,uterine fibroids and kidney tumors,showing high safety and effectiveness^[1-3].The use of HIFU for ablation treatment can achieve better ablation effect under the premise of ensuring safety,but clinical treatment relies mainly on the experience of doctors to subjectively determine the irradiation dose^[4].Therefore,dose delivery during HIFU treatment remains a key scientific issue to be resolved.For the establishment of the HIFU therapeutic dose system,the acoustic propagation and bioheat transfer models are still an important part of the experiment and simulation,and further research is still needed.This study focused on the scientific issues of HIFU clinical dose delivery prediction,through exploratory research on transient acoustic propagation at focal ultrasound focal region and bioheat transfer related mechanisms,in order to optimize the sound field for clinical treatment to choose the appropriate angular angle transduction Provide guidance to optimize the acoustic propagation and bioheat transfer models during HIFU irradiation to provide a reference for the establishment of clinical treatment dose prediction systems.Objective:1.To carry out the visualization study of transient acoustic propagation in the focused ultrasonic focal region,to explore the acoustic propagation characteristics of different angular cavity transducers,and to provide reference for the optimization of the opening angle of the transducer.2.Consider the thermal wave effect of tissue heat transfer during HIFU irradiation,the acoustic characteristics of tissue（sound velocity,sound attenuation）with temperature rise,and compare the temperature rise changes and theoretical simulations in the target tissue under the same irradiation dose.Optimize the acoustic propagation and bioheat transfer models for HIFU treatment.Method:1.This paper will study the acoustic propagation in the focal region of a traveling wave HIFU transducer with different opening angles.The664.2KHz HIFU transducer with an opening angle of 360°,180°,150°,120°,90°and a focal length of 115 mm is simulated by COMSOL at the same excitation voltage for its steady-state and transient sound field distribution,based on the schlieren method captures the sound propagation in the focused ultrasound focal region.Real-time shooting and simulation analysis of the distribution of the sound field during the traveling wave focusing process and the change of the focal length.2.In this paper,the traditional Pennes bioheat transfer equation is modified based on the concept of finite heat propagation velocity,and the thermal wave model of bio-heat transfer（Twmbt）model in HIFU irradiation is established.Further research is carried out.The thermoacoustic lens effect of sound propagation in the tissue,when t he temperature is higher than 45°C during irradiation,the tissue char acteristics begin to change gradually,and the sound field-temperature field coupling model of tissue sound velocity and sound attenuation with temperature changes during irradiation is established.Theoretica l simulation;the influence of the grid setting on the sound axis with temperature on the sound field and temperature field is also discuss ed.3.In this paper,the experimental study systematically studied th e temperature rise of the tissue focal region under the same irradiati on dose of different irradiation parameters（1039.95W/cm²×267s,1386.6W/cm²×200s）when HIFU fixed-point irradiation of fresh isolated bo vine liver tissue.Using B-ultrasound to monitor strong echo signals,the PCD probe collects acoustic scattering signals for passive cavitati on detection to ensure that the mechanism of HIFU irradiation of the temperature rise of isolated bovine liver tissue is a simple thermal effect.The bioheat transfer model was further revised by theoretical and experimental comparison analysis.Results:1.As the angle of the transducer opening increases,the axial-6dB focal region decreases,and the focal region shape gradually cha nges from elliptical to circular.When the opening angle is 360°,the-6dB acoustic focal region has the smallest area and is“circular”;when the opening angle is 90°,the-6dB acoustic focal region has t he largest area and is“elliptical”.With the increase of the opening a ngle（90°,120°,150°,180°,360°）:（1）The long axis of the sound f ield of the hydrophone sound field scanning is 2.39λ,2.02λ,1.58λ,λ,0.53λ,corresponding to the linear sound field simulation results 2.37λ,2.16λ,1.63λ,1.15λ,0.55λ;（2）the short axes are 0.85λ,0.71λ,0.59λ,0.55λ,0.52λ,respectively,corresponding to the linear sound fi eld simulation result 0.9λ,0.72λ,0.62λ,0.56λ,0.51λ;（3）-6dB acous tic focal regions are 3.47λ²,2.49λ²,1.61λ²,0.95λ²,0.48λ²,respectivel y,corresponding to linear sound field simulation results 3.69λ²,2.69λ² 1.77λ²,1.1λ²,0.49λ².The hydrophone scanning experimental results agree well with the Comsol steady-state sound field distribution sim ulation results,and the long-axis,short-axis and-6dB axial acoustic f ocal area correlation coefficients are 0.995,0.992,0.999,respectively,all reached 0.99 or above.2.With the increase of the opening angle,the faster the ultrasonic radial focusing width decreases during the ultrasonic propagation in the-6dB acoustic focal field,the stronger the focusing ability,and it is obvious from the result graph that when the opening angle is constant,the ultrasound When the convergence reaches the-6dB focal length position,the radial width of the acoustic wave remains basically the same,similar to the forward propagation of the plane wave,the ultrasonic continues to propagate forward,and the far-field sound wave is gradually lost;the larger the opening angle,the smaller the radial width.3.In the case of simple thermal injury,HIFU irradiates the isolated bovine liver tissue,and the same irradiation dose,with the increase of the irradiation intensity,the faster the temperature rise rate.Using the Pennes bioheat transfer equation theory to simulate the HIFU fixed-point irradiation（irradiation parameters of 1039.95W/cm²×267s and1386.6W/cm²×200s）,the initial stage of irradiation（0⁴8s for1039.95W/cm²×267s,0⁵7s for 1386.6W/cm²×200s）,the focal temperature rise is significantly higher than the experimentally measured temperature rise;the temperature rise of the focus simulated by the TWMBT model agrees well with the experiment in the initial stage of irradiation,but irradiationMediummaximumtemperaturerise（27°Cfor1039.95W/cm²×267s,36.5°C for 1386.6W/cm²×200s）compared with experimental measurements（32.5°C for 1039.95W/cm²×267s,41.5°C for1386.6W/cm²×200s）Low;the temperature rise of the focus（32°C for1039.95W/cm²×267s,42°C for 1386.6W/cm²×200s）considering the thermoacoustic lens effect and the TMWBT model is in good agreement with the experimental results throughout the HIFU irradiation process.Conclusions1.When the driving power is constant,the-6dB acoustic focal range gradually decreases with the increase of the opening angle,and the more concentrated the energy distribution,the shape gradually changes from“elliptic”to“circular”;the correlation coefficient between experimental and comsol simulation results are both above 0.95.When the opening angle is fixed,the ultrasonic propagation convergence reaches the-6dB focal length position,and the radial width of the acoustic wave remains basically the same.Similar to the plane wave propagating forward,the ultrasonic wave continues to propagate forward,and the far-field sound wave is gradually lost.2.When the HIFU irradiation energy is constant,the faster the focal temperature rises with the increase of sound intensity;under the condition of pure thermal damage,comprehensively consider the acoustic characteristics of biological tissue with temperature change,thermal wave effect,sound axis direction network Based on the influence of grid setting and other factors,a suitable model of tissue acoustic properties（sound velocity,sound attenuation）with temperature is established to simulate the change of focal temperature rise of HIFU irradiated bovine liver tissue,and the tissue acoustic characteristics are considered after correction.The modified Twmbt bioheat transfer model is better than the experimental Pennes bioheat transfer equation.