Motion Modeling And Path Planning Of Cannula Flexible Needle Based On Finite Deformation Theory

Minimally invasive interventional surgery is an emerging novel medical subject that has developed rapidly in recent years and is an inevitable trend in the development of clinical surgery.Targeting insertion is one of the important means of minimally invasive interventional surgery.A cannula flexible needle consists of a flexible cannula and a flexible stylet with a bevel tip.The curvature of the insertion path can be changed by adjusting the distance of the stylet out of the cannula,thereby to correct the path,to bypass the obstacle,and to achieve a precise targeting control in the deep portion of the human body.In this dissertation,the key issues of the robot manipulated cannula flexible needle for targeting insertion are focused on,and the stress and bending are deeply studid of the cannula flexible needle inserted into the human tissue.Furthermore,the path planning and experimental research are carried out providing theoretical and experimental basis for the precise control of the position,posture,trajectory and target insertion of the cannula flexible needle.Force analysis of the cannula flexible needle in the process of insertioin into the soft tissue is performed deeply,and the force acted on the needle is determined as cutting resistance and cutting friction acted on the bevel tip,clamping force,clamping friction,tissue resistance,tissue resistance friction,supporting force and supporting friction acted on the needle shaft.Forces are modeled,measured and calculated,and the relationships between them are established,thereby a unified force system is obtained.The resultant force of all forces in the direction of the needle at the root of the needle shaft as well as the supporting force and supporting friction are measured by the combination usage of a single-dimensional force sensor and a six-dimensional force sensor,and the decoupling measurement of the forces is realized.Moreover,The unknown forces are solved by the experimental data and the mechanical properties of the needle material.This lays a theoretical foundation for the bending modeling of the cannula flexible needle based on mechanics.Based on the mechanical models of the cannula flexible needle,the finite deformation theory and the cantilever beam theory are combined to establish the bending model of the cannula flexible needle inserted into the soft tissue according to the mechanical properties and the situation of the forces for the needle.By discretizing the model,the dynamic direction change of the force acted on the needle tip is realized,i.e.,the cutting resistance changes with the bending of the needle shaft(or the posture of the needle tip),which more accords with the actual loading situation and deformation of the cannula flexible needle.Furthermore,the results of the bendings under different discrete lengths are analyzed,then the appropriate discrete length and bending which is consistent with the actual trajectory are obtained,which proves the rationality and accuracy of the proposed model.Based on the kinematic model of the cannula flexible needle,the multi-tree search strategy,target guidance strategy,reachable space strategy and optimal selection strategy are embedded into the traditional Rapidly-exploring Random Tree(RRT)algorithm,and an improved RRT algorithm(I-RRT)suitable for the path planning of the cannula flexible needle is proposed.Based on the I-RRT algorithm,the intraoperative dynamic path planning algorithm is proposed,and three improvement strategies are proposed: Old Point Tracking Strategy(OPTS),Extreme Trend Extention Strategy(ETES),Hardest Goal Tracking Strategy(HGTS).They are embedded into the program of the basic dynamic path planning algorithm to solve the big detour problem,convergence problem and precision problem of the basic dynamic algorithm,respectively.Simulation results show that the algorithm has the advantages regarding to computation speed,convergence,form of the optimal path and robustness of the searching.Nitinol(nickel-titanium alloy)wire and tube are adopted to manufacture the cannula flexible needle.After the analysis and comparison of the phantom tissues for the human tissue,agar is finally selected and prepared.The experimental system assisted by robot for the cannula flexible needle insertion is established,and a series of experimentations are conducted,including the needle-tissue friction coefficient measurement experiment,insertion force experiment,bending experiment and the planned path verification experiment.The measurement methods of the needle-tissue friction coefficient and the decoupling measurement of insertion force are proposed,and the rule of bending of the needle is obtained under different lengths of stylet out of the cannula.The results of path planning verification experiments prove that the systematic error of the insertion is within 3%,which verifies the accuracy and effectiveness of the proposed curvature changing functions.