【作者】 徐少平；

【导师】 张华； 刘小平；

【作者基本信息】 南昌大学 ， 机械电子工程， 2010， 博士

【摘要】 在虚拟手术仿真系统中,软组织形变模型主要用于描述人体器官软组织和手术器械之间的交互过程,它直接决定了手术仿真系统中视觉反馈和力觉反馈的精度、速度和仿真效果。由于软组织是一种具有线性、非线性、不可压缩性、各向异性和粘弹性等复杂性质的特殊弹性材料,解决软组织形变模型的视觉/触觉逼真度和计算实时性之间的矛盾是虚拟手术仿真系统当前重要的研究课题。本论文从研究解决以上难题为根本出发点,主要完成了如下一些研究工作：通过引入松弛函数(Relaxation Function)和迭代更新(Iterative Update Scheme)计算方法,基于Tensor-Mass模型(Tensor-Mass Model-TMM)提出了一种改进后的新模型(Improved Tensor Mass Model-ITMM)。ITMM模型主要特点是在原模型的基础上增加了对软组织粘弹性的描述能力,使得它可以比较全面地描述软组织各种材料性质,同时却没有明显地增加计算量,仍然可以满足实时交互要求。通过反求优化算法(Inverse Optimization Algorithm),将ITMM模型仿真的数据与软组织挤压实验数据进行了对比分析,测试结果表明：在经过优化算法找到合适的模型参数后,ITMM模型具有很好的数据拟合能力,准确性高。同时,在一个虚拟肝脏手术仿真系统中,通过蠕变、松弛和Hysteresis等测试项目的检测进一步验证了ITMM模型的性能。与TMM模型一样,ITMM模型实际上也是由弹簧质点模型和有限元模型构成的混合模型。由于它对应变能函数(Strain Energy Function-SEF)定义相对简单,尽管ITMM模型在TMM模型在增加了软组织粘弹性的描述能力,ITMM模型描述软组织不可压缩性、各向异性等材料性质的能力仍有待提高。为进一步提高ITMM模型的性能,提出了一种全新基于弹簧质点模型和有限元模型的混合实时形变模型(Hybrid Real-time Deformable Model-HRDM)。HRDM模型是通过重新定义ITMM模型的应变能函数定义得到的,可以看成是ITMM模型的一般化推广。它完全按照连续介质弹性理论的方法导出节点内力计算公式,而在求解运动方程时候则采用与经典的弹簧质点模型相同的方法。因此,HRDM模型可以比较全面的描述软组织各种材料特性并且保持了弹簧质点模型计算结构简单、内存需求少和计算效率高的优势。HRDM模型参照ITMM模型验证和测试方法进行类似的测试,实验仿真数据表明HRDM模型在对软组织材料性质描述能力较ITMM模型更优,运行效率上达到了实时交互的要求。为了能够描述软组织表面微观性质,本文提出了一种直接基于视觉纹理图像利用保持边缘特征的彩色图像降噪算法获得触觉纹理(Haptic Texture)的方法。通过基于触觉纹理的微观力反馈渲染算法以摩擦力的形式进一步提高HRDM模型对手术器械和软组织表面交互的渲染效果。这种微观力反馈渲染算法不需要专门设计力反馈硬件设备,仅仅使用通用力反馈设备即可。另外,它最大的特点就是实现视觉和触觉线索紧密融合,到达了“所见即所触”的效果,使得在虚拟手术环境下操作者获得的沉浸感更强。手术的过程实际上就是医生的手与眼睛相互协调,共同完成特定操作任务的过程。在虚拟手术仿真系统中提供立体显示的功能,将能在视觉上提供真正的深度感觉,提高训练效果。本文在分析各种立体显示相关技术原理的基础上,提出了一个虚拟手术仿真系统立体显示子系统实现方案。该方案最主要的优点就是它的软硬件兼容性好,对现有程序改动量小和性价比高。将本文所研究成果应用到一个气管切开虚拟手术仿真系统中。对该系统的设计与实现细节以及主要功能进行了简要介绍。更多还原

【Abstract】 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.更多还原