| The liver is prone to deformation,dense blood vessels and complex internal tissue structure,and the internal tissue structure of the liver can not be observed directly during the operation,which puts forward very high requirements for doctors’ surgical skills.With the help of virtual surgery system,doctors can be well trained in surgery and improve their surgical skills.However,at present,virtual liver models are set as homogeneous models,and doctors can not accurately perceive the deformation of the internal structure of the liver.According to the heterogeneous characteristics of the liver,this paper obtains the follow-up deformation of blood vessels and tumors in the liver during liver deformation,so as to provide doctors with more intuitive perception ability and improve the level of surgical skills.Moreover,the study of heterogeneous liver deformation has great application prospects and value for preoperative surgical planning,intraoperative surgical navigation and education and training.At present,there are two main problems in liver soft tissue deformation modeling algorithm,namely,real-time and accuracy.Finite element algorithm is a soft tissue modeling algorithm with high deformation accuracy,but its real-time performance is very poor due to complex calculation,which is only suitable for off-line calculation.This paper solves the real-time problem of finite element deformation modeling algorithm by deeply studying and analyzing the principle of finite element algorithm and optimizing algorithm structure and data structure.Aiming at the heterogeneity of liver soft tissue,a mechanical coupling algorithm between liver parenchyma and liver blood vessels is proposed to realize the deformation prediction and visualization of liver internal structure.On this basis,the collision detection algorithm and force feedback calculation in the virtual surgery system are studied and analyzed.Finally,a virtual surgery system based on heterogeneous liver model is built.Firstly,this paper completes the research on liver soft tissue modeling.Based on the analysis of the biomechanical characteristics of liver soft tissue,Neo-Hooken is selected as the constitutive equation of the soft tissue physical modeling algorithm in this paper;According to the CT image reconstruction,the liver soft tissue surface model based on triangular mesh is obtained.Combined with the open source tetrahedralization tool tetgen.the tetrahedralization of the geometric model of liver and blood vessel is completed;The physical modeling of liver soft tissue is completed based on the finite element algorithm of Lagrange discretization.The computational complexity is simplified by explicit integration algorithm and calculating the internal force of element nodes from the element level to avoid the calculation and assembly of stiffness matrix.The real-time problem of finite element deformation modeling algorithm is solved and the modeling of liver soft tissue is completed.Secondly,the heterogeneity of liver soft tissue is studied.Set the boundary conditions between the liver parenchyma and the liver vein,construct a reasonable node mapping relationship,and calculate the internal force of the tetrahedral element node in the contact area by realizing the coupling calculation and updating of the node displacement,deformation gradient matrix and strain displacement matrix,so as to complete the mechanical coupling between the liver parenchyma and the liver vein.Considering the amount of data of liver soft tissue nodes and the great demand of deformation modeling algorithm for data processing ability.GPU is used to accelerate the calculation,which effectively meets the requirements of computing power and real-time performance of the system.Then,in order to realize the interactive operation of liver soft tissue model in virtual surgery system,this paper studies the collision detection algorithm and force feedback calculation model respectively.A hierarchical bounding box tree based on the surface area heuristic of AABB bounding box is built,and the data structure of the bounding box tree is optimized and transformed into a bounding box array to improve the traversal efficiency;Secondly,the force feedback calculation method based on mass spring algorithm is adopted to complete the construction of the force feedback calculation model in this paper by setting agent points,which has high calculation efficiency and improves the real-time performance of the virtual surgery system.Finally,based on the research of heterogeneous liver soft tissue deformation modeling algorithm,a virtual surgery system is built.The whole system is divided into three modules:visual rendering,deformation modeling calculation and force tactile rendering.Geomagic Touch is used as force feedback equipment,and the virtual surgery system is designed and built based on unity and other software;Secondly,based on unity’s own graphical interface development tool UGUI,the interactive interface of virtual surgery system is designed and realized;Finally,a set of virtual surgery simulation system based on heterogeneous liver is built,and the gravity deformation experiment of homogeneous liver,heterogeneous gravity deformation experiment and heterogeneous liver deformation tensile experiment are carried out.The experimental results show that the heterogeneous liver deformation modeling algorithm and virtual surgery system have high real-time and authenticity. |
