| High intensity focused ultrasound(HIFU)is a new tumor treatment technology with great potential,which has been widely used in clinical treatment of tumors.Because it has many advantages such as non-invasive,non-toxic side effects and has attracted more and more attention in recent years.As a non-invasive treatment technology,HIFU can produce local high temperature in a short time,making the tumor tissue in the target area undergo coagulative necrosis without causing damage to the surrounding normal tissue,achieving the goal of tumor ablation.Before the HIFU treatment,treatment planning is an indispensable part,which can provide the guidance for HIFU treatment,and the numerical simulation is an important tool for predicting the efficacy of treatment,which can help doctors adjust the treatment plan in time.Using reliable numerical simulation to accurately predict the temperature and thermal damage in the lesion area is the key to ensure the success of treatment.Based on the finite ultrasonic propagation model and the bioheat transfer model,three works have been carried out in this paper around the accurate prediction of temperature and thermal damage in the lesion area:(1)Considering the non-Fourier characteristics of heat conduction in biological tissues,the effects of blood vessels on HIFU heating were studied;(2)The effects of dynamic tissue properties on HIFU heating were studied;(3)The temperature prediction of multi-vesselbiological tissues during HIFU irradiation was studied.The specific work is as follows:Firstly,Most of the studies on thermal effects in biological tissues are based on Pennes bio-heat transfer model,which is based on Fourier law of heat conduction,assuming that biological tissues are uniform and the heat transfer speed is infinite.However,heat conduction in biological tissues has non-Fourier behaviour.Therefore,the Pennes bioheat model was modified to thermal wave model of bioheat transfer(TWMBT),considering the effect of blood vessels on HIFU heating,and a joint physical model combining TWMBT model(perfusion tissue)and heat energy transfer equation(vascular)was proposed.The effects of Pulsatile blood flow,thermal relaxation time,acoustic focus location,vessel radius and blood velocity on the temperature and thermal damage of tumor in the lesion area during HIFU heating were studied.The research results showed that:(1)Thermal relaxation time has a great effect on the temperature and the thermal lesion region.Larger thermal relaxation time results in lower peak temperature and smaller thermal lesion region,which indicates that Pennes bioheat transfer model overestimates the peak temperature and thermal lesion region compared with TWMBT model.(2)The ultrasonic focus location and blood vessel radius significantly affects the peak temperature and thermal lesion region.The greater the distance between the acoustic focus and the center of the vessel,the higher peak temperature and the larger the thermal lesionregion.The larger blood vessel radius results in lower peak temperature and smaller thermal region,and the thermal lesion region is very sensitive to the blood vessel radius.(3)The heartbeat frequency and amplitude factor of pulsating blood flow as well as the average velocity of blood flow had just a slight effect.Secondly,the temperature rise of tissue induced by HIFU irradiation will cause changes in the tissue properies that affect the acoustic and temperature field in turn,and finally the tissue propery parameters,acoustic field and temperature field interact with each other during HIFU irradiation.For the defect that the traditional numerical prediction method usually sets the tissue propery parameters as constant,an acoustic-thermal coupling model was presented to predict the temperature and thermal lesion region in tissue in terms of the Westervelt equation and Pennes bioheat transfer equation,considering the change of tissue propery parameters during HIFU irradiation.First,based on the experimental data of temperature-dependent tissue propery parameters measured by some researchers,the relationship between tissue propery parameters and temperature was obtained by polynomial fitting.Again,the fitting Polynomial was used to study the effect of the change of single parameter of tissue propery and the combined effect of all dynamic tissue property parameters on HIFU heating.The simulation results showed that:(1)Sound pressure is not significantly affected by the dynamic changes of tissue properties;(2)Among all of the individual dynamic tissue propertyparameters,the dynamic acoustic absorption coefficient had the greatest influence on the temperature and thermal lesion region,while the dynamic non-linear parameters,dynamic acoustic velocity,dynamic specific heat capacity and dynamic density have less influence;(3)Compared with the conventional methods,the dynamic acoustic absorption coefficient leaded to a higher focal temperature and a larger thermal lesion region;on the contrary,the dynamic blood perfusion leaded to a lower focal temperature and a smaller thermal lesion region.(4)Compared with the traditional methods,the predicted maximum focal temperature is higher when considering the change of thermal conductivity,however,the thermal damage predicted by the two methods is almost equal.(5)The conventional method underestimated the focal temperature and thermal lesion region,compared with the simulation that was performed using all of the dynamic tissue property parameters.Thirdly,in order to predict the temperature of multi-vessel biological tissue during HIFU irradiation,the biological tissue was treated as porous medium in this paper,and a generalized Dual-phase lag bioheat transfer(GDPL)model was proposed to study the heat transfer behavior of HIFU-irradiated multi-vessel biological tissues.Compared with Pennes bioheat transfer model and TWMBT bioheat transfer model,and the effect of porosity on HIFU heating was also studied.The simulation results showed that:(1)compared with Pennes model and TWMBT model,when the porosity was small,the temperature and thermal lesionregion predicted by the GDPL model were in good agreement with the other two models;When the porosity was large,the temperature and thermal lesion region predicted by the GDPL model was smaller than that predicted by the other two models.(2)The larger the porosity had,the more blood vessels were contained in the tissue,the stronger the heat convection between the blood vessel and the surrounding tissue were,the lower the temperature of the tissue had,and the smaller the thermal lesion region was.(3)The greater the porosity had,the greater the temperature difference between tissue and blood was.In the heating stage,the temperature of tissue rised faster than that of blood;and in the cooling stage,the temperature of tissue decreased faster than that of blood.The results of this study will be helpful to guide doctors to formulate more accurate clinical implementation plan for HIFU treatment,and further promote the clinical application of HIFU. |
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