The Real-time Deformable Model Of Soft Tissue For Surgery Simulation

The key component of surgery simulation systems is a realistic soft tissue model which has been developed in an attempt to depict interactions between soft tissue and surgical instruments in the virtual environment. The deformable model of soft tissue determines directly the accuracy, speed and effectiveness of both visual feedback and force feedback. Although a significant amount of research efforts have been dedicated to simulating the behaviors of soft tissue, modeling of soft tissue deformation is still a challenging problem due to the complicated linear, nonlinear, anisotropic, nonhomogeneous, and viscoelastic (time, and rate dependent) behaviors of soft tissue. The most difficult problem is to satisfy the conflicting requirements of real-time interactivity and visual/haptic realism. In this thesis, studies were carried out in the following aspects:By introducing the relaxation function, internal variables and the iterative update scheme, we proposed an improved Tensor-Mass Model(ITMM). ITMM model is characterized by increasing the model's ability to describe the viscoelasticity of soft tissue, making it have much better description of various material properties of soft tissue. In the meantime, the new model does not compromise much the computation efficiency of original tensor-mass models. ITMM still meets the requirements of real-time interaction. The developed constitutive model was validated by means of the inverse optimization algorithm based on in vitro experimental data measured on cubic tissue samples for which the simulated annealing algorithm was used to obtain numerical values for the biomechanical parameters in compression tests. The quantitative comparison with the experimental results shows that the developed model resembles the real data very well. The proposed model was also used to simulate human liver in a prototype of surgical simulators.Like TMM model, ITMM model is actually a mixed model based on the Mass-Spring model and finite element model. Due to its relatively simple definition of strain energy function, its descriptive capability of the material properties, such as incompressibility and anisotropy, is very limited although ITMM model is able to incorpate viscoelasticity. We present a hybrid real-time deformable model (HRDM) in which the internal forces among mass nodes are derived within the framework of continuum mechanics. With the potential (strain energy) function of a classical mass-spring model being replaced with the new proposed one, the new model is able to describe typical behaviors of living tissue such as incompressibility, nonlinearity and anisotropy. In addition, the time-dependence of soft tissue (viscoelasticity) is also considered in the new model. Our approach is distinct from conventional mass-spring models in that a modified strain energy function, in accordance with three-dimensional finite strain elasticity theory, is used to derive internal forces among mass points. The new model can still keep the advantages of the mass-spring model such as a simple architecture, low memory usage, and fast execution speed. In addition, it has strong biomechanical relevance compared with the convention mass-spring model. The capabilities of the model and the efficiency of implementation are assessed by the benchmark problems and human organ simulation.In order to describe the microscopic surface properties of the soft tissue, this thesis presented a simple and intuitionistic algorithm of haptic texture generation based on image processing. As such, we proposed a new haptic texture rendering approach which can be called the macro force disturbance rendering algorithm. The approach requires minimal hardware support, and can be implemented on the standard force-feedback mechanism like PHANTOM Omni. The main advantage of our tactile rendering technique is that we can combine visual clue with haptic clue to make the surface of object feel more realistic.The operation process is mainly the coordination of the surgeon's hands and eyes. Providing three-dimensional display function in the virtual surgery simulation system will be able to provide real depth in the visual sense and improve training effect. This thesis analyzed a variety of related technologies about binocular stereoscopic display and proposed a framework of real-time rendering and displaying stereoscopic scenes of surgery simulation. The main advantage of the framework is good compatibility and low expense.The research results in this thesis were applied to a open cricothyroidotomy virtual surgery simulation system. The design, implementation and main features of the simulation system were prsented in the last chapter.