Research On Viscoelasticity Soft Tissue Modeling And Cutting And Virtual Simulation Experiment

The surgical training mode offered by virtual surgical simulation system has no damage, strong repeatability and immersive. Virtual surgery simulation system can not only reduce the cost of training, implement training resources reuse, but also can eliminate risk operation training. In recent years, the cirtual surgical simulation system is becoming increasingly wider and deeper. The soft tissue modeling, collision detection between irregularity objects, image processing and visual rendering are the key of improve the precision and effectiveness of the virtual surgery simulation system. Based on these results, this paper studies soft tissue modeling, multi-model collision detection in complex space, image segmentation method and blood simulation, and it will improve the precision, real-time and interaction of system.According to the characteristic of mesh soft tissue modeling and mesh-less soft tissue modeling method at present, this paper establishes a new hybrid viscoelastic soft tissue model based on three parameters and spheres-filled. This model consists of a layered structure and many spheres-filled. Each of the two spheres is connected by a three parameters viscoelastic structure. To verify the precision of our virtual soft tissue model, we should get the viscoelastic data of real organ. A measurement platform of soft tissue organ was designed and built in this paper. Compared with the real organ experimental data, this viscoelastic model not only shows the force feedback character but also has the better stress-strain, stress relaxation and creep properties. Compared with mass-spring model, the simulated results show it has better creep, stability and accuracy.In view of the different soft tissue, this paper presents surface cutting algorithm and volume cutting algorithm. Incision is formed by moving the vertex of traingle in surface cutting method, and getting the smooth incision by using Bezier curves. According to the intersection between cutting line and triangle, we copy the intersection and creat sub-traingle, and link the sub-traingle and the position of cutting depth to form incision. These methods need no rebuild mesh, and ensure the real-time and smooth incision. The cutting method of spheres-filled of surface and volume cutting is based on spheres breakage, and separate spheres forming incision. In order to achieve soft tissue cutting simulation, we should get the collision detection between irregularity geometry virtual surgical drill and soft tissue model. A novel algorithm for collision detection in virtual operation training system based on octree of adaptive subdivision and encodin g algorithm was proposed in this paper. Using this method, this structure can not only guarantee the real-time, and has the good topology. This octree structure adopts three times of the average triangle of the axis aligned bounding box to set the cell size. It is reasonable and effective compared with the radius of sphere. Based on the triangle size and the optimal cell size, by using the adaptive triangular mesh subdivision, and the centroid distance, the collision in the space was detected. The experimental simulation data verify the real-time and effectiveness of the method.There is a lack of targeted training for different organ cases of patients in doctor’s operation training. First, an adaptive weighted median image filtering was proposed for medical image noise in this paper. Second, we use a pre-segmentation method based on optimal threshold ant and fuzzy clustering algorithm to segment the filtered image. Third, an improved semi-automatic GVF-Snake medical image segmentation algorithm of constraint contour line for pre-segmented image was proposed in this paper. The algorithm was validated using the diseased liver CT scans image to attain the better distribution of contour points and the higher precision of contour area, thereby validating the effectiveness of algorithm.Last, we apply modeling method, cutting algorithm, blood simulation and collision detection to a virtual operational soft tissue simulation system. Using the force feed-back device, we complete the palpation and cutting simulation in virtual surgery system, the real-time feedback data verify the effectiveness of collision detection and viscoelastic of soft tissue after touch or cutting simulation.Meanwhile, in view of the feature of soft tissue cutting simulation, a variable range-based smoothing length of Smoothed Particle Hydrodynamics(SPH) method with variable smoothing length is proposed based on fix smoothing length SPH. The method can make the gradient of density smaller. The simulation results verify the proposed method has better press and viscous force in virtual organ cutting experiment.