Prediction Algorithm And The Verification Of Phase Change Temperature Field Based On Spherical Coordinates In Cryotherapy

Cryotherapy has the advantages of minimally invasive,low cost of treatment,and has lower chance to relapse after the treatment.It is widely used in the treatment of tumors and other diseases.Cryotherapy achieves therapeutic goals by inactivating the diseased tissue cells at low temperatures.At present,the academic research on cryotherapy mostly focuses on the qualitative analysis of the actual surgical procedure.For the prediction of temperature field changes,there is no simple and accurate calculation method.In this paper,a simple and fast temperature field prediction algorithm based on the moving heat source method and spherical coordinates is proposed for the most commonly used cryotherapy probe.The main work of this paper is as follows:(1)The temperature dependence of metabolic perfusion and blood perfusion rate is studied.And the relationship between thermal capacity and thermal conductivity of tissue and temperature are studied in the Pennes heat transfer model.On the basis of certain simplification,the differential equation of biothermal conduction in spherical coordinates is proposed using the moving heat source method to treat the latent heat in phase change.(2)Based on the biothermal differential equations in spherical coordinates,the prediction algorithms under steady state and transient state are proposed respectively.By introducing the steady-state boundary conditions,the temperature field prediction formula of the biological tissue with the unknown parameter freezing radius is derived,and then the heat balance equation brought into the phase change interface finally derives the analytical formula of the freezing radius,and then obtains the complete steady-state temperature field prediction algorithm.The steady-state algorithm results are used as the terminal conditions derived from the transient algorithm,and the prediction algorithm under transient conditions is obtained by establishing the thermal equilibrium relation of the phase-change boundary layer.Finally,the method of correcting the freezing coefficient according to the freezing rate of different cryotherapy probes in the transient algorithm is given.(3)Through the steady-state numerical simulation of the temperature field during the freezing process of cryotherapy,the applicability and accuracy of the prediction algorithm are discussed.Firstly,the temperature field distribution of cold source at different depths in biological tissues is simulated,and the influence of convective heat transfer on the surface of the tissue and air on the temperature field is studied.Then,the temperature field distribution under the condition of spherical symmetry is simulated,and compared with the prediction results of the prediction algorithm,the overall influence degree of various simplifications on the results in the prediction algorithm is studied.(4)The ex-vivo freezing experiment of biological tissues is carried out.The temperature variation of the temperature points of biological tissues during the freezing process is obtained by experiments,and compared with the steady and transient state prediction results of the prediction algorithm,to verify the accuracy and reliability of the algorithm.The freezing experiments of two kinds of freezing rate cold knife probes is carried out by stabilizing the pressure in the tank at 0.04 MPa and 0.08 MPa.After that,the experimental results and the predicted results are compared and analyzed to verify the accuracy of the pre-tests on the actual situation.(5)Compile the prediction algorithm into a windowing software for the computer by program code compilation.By discussing the software requirements,the program functions are divided into the main system,the steady state prediction system and the transient prediction system,and the flow chart of each system is listed.Then the realization of the key code of the software to realize the prediction calculation corresponding to each function is discussed.Finally,the software's error handling problem is studied,so that the program can run safely and stably.Through the preparation of cryotherapy prediction software,the combination of theoretical analysis and practical application is fundamentally realized,which provides an easy-to-use predictive form software for cryotherapy workers,and provides a basic framework for the subsequent software function improvement.