Research On The Tearing And Bleeding Of The Soft Tissue In Virtual Surgery

Virtual surgery simulation system is the application of virtual reality technology in the field of Medicine.Compared with the traditional methods of training surgeons,virtual surgery has a series of advantages such as low cost,high efficiency,free from time and space constraints.The virtual surgery simulation system establishes a virtual environment similar to the real surgery scene through computer technology.In this environment,surgeons can make and perfect preoperative plans,projecting intraoperative paths,cultivating surgical skills,practicing squeezing,stretching,cutting,resecting and other operations in the real process of surgery,and do suction treatment for intraoperative bleeding.At present,the research of virtual surgery has attracted the attention of scholars at home and abroad.Different types of systems and functional modules are improving day by day.The tearing module and the bleeding module require high real-time and fidelity.In real surgery,the tearing and bleeding of soft tissue often occur together.The location of tearing and the destruction of blood vessels directly affect the state of bleeding.However,the different physical models make it difficult to connect the two modules.In view of the above problems,in this paper,improved tearing model and bleeding model were proposed.Taking the cutting simulation of liver tissue as an example,the combination of the two modules in the virtual surgery simulation system was realized.The fidelity of visual effect and haptic interaction was improved,and the immersion of users was enhanced.The main research work is as follows.The three-dimensional models of soft tissues that can accurately express the internal structure were established.In view of the lack of internal information in the reconstruction method based on surface rendering,the improved ray casting algorithm is proposed for 3D reconstruction of soft tissues.Volume data is extracted from CT / MRI images of real patients and preprocessed to obtain multidimensional parameters consisting of gray value,density value,gradient value and second derivative value.The function of each parameter is analyzed qualitatively and quantitatively through the section color difference models and the sampling value curves.The multi-dimensional transformation function instead of the one-dimensional transformation function in the traditional algorithm is used to complete the mapping of color information,improving the discrimination between structures and the clarity of boundary,and accurately expressing the internal structure information of soft tissues.Through GPU parallel computing and compact bounding box,the software and hardware are combined to increase the calculation speed,and the rendering time meets the real-time requirements of simulation.A tearing model of soft tissue based on improved geometric model and meshless method is proposed.The function relationship between the multidimensional parameters and the coefficients of point element is established to realize the combination of geometric model and physical model.In the cutting process,the change of the model is calculated according to the magnitude and direction of the external force applied by the surgical instrument to the soft tissue.The position information of the computational points and the parameter information of the corresponding volume data points are obtained in real time.When the incision is generated,the second-order Bézier curve is used to optimize the boundary without reducing the real-time performance of the simulation.The precise internal structure of soft tissue can be seen from the incision.According to the multidimensional parameter values corresponding to the computational points,the structural types of the position can be determined.The corresponding haptic interaction can be provided to the surgical instrument.The user can judge the tissue touched and damaged in the operation process from the visual and haptic aspects at the same time,so as to establish the basis for the geometric connection between the tearing and the bleeding modules.A bleeding model based on the improved Navier Stokes(N-S)equations solved by the improved smooth particle hydrodynamic(SPH)algorithm is proposed.The calculation of the tension is introduced into the pressure term when the bleeding particles are on the model surface.In the calculation of viscosity,the influence of platelet is introduced,and the gathering speed of viscosity increases with time.The core radius of the SPH algorithm is changed to be a function of the density of particles.The repulsion force between particles is introduced,and it is calculated by the Lennard Jones form.The improved bleeding simulation has the viscosity and coagulability which are consistent with the real blood.The edge distortion caused by the forces on the boundary of bleeding particles tending to 0 and the volume distortion caused by the penetration between particles are avoided.The improvement of physical model is instrumental in the fidelity of bleeding simulation.The Marching Cubes(MC)algorithm and the Mixed Depth Rendering(MDR)algorithm are respectively used to render the simulation of violent gushing and slow flow bleeding.In the cutting operation,if the important blood vessels are borken,the blood will be produced at a large initial speed,showing the effect of gushing.For the bleeding in this state,the MC algorithm is used to render,and achieves a fine three-dimensional visual effect.When the important blood vessels are not broken,the blood will slowly produce from the incision and spread to the surrounding area,mainly in two-dimensional visual presentation.In this paper,the MDR algorithm is proposed.It has good visual effect and computational efficiency to render slow flowing bleeding simulation.A suction model suitable for MDR algorithm is proposed to realize the simulation of bleeding suction.Finally,based on the above work,the combination of tearing and bleeding modules in virtual surgery simulation system is realized.From the test results,tearing and bleeding in the system establish a reasonable geometric connection.When users operate correctly,the suction operation can be carried out.The visual and haptic fidelity and real-time performance of simulation are improved.