Numerical Simulation Study On Aneurysm And Stent Based On Reverse Engineering

With the development of technology, it is very popular to use stent to treat vascular aneurysm. Many factors, such as stent structure, cross-section of stent wire, porosity, placement of stent, shape of stent connector and fatigue characteristics of stent will have different influence on stent intervention of aneurysms. It hasnot been reported about what different impacts on the hemodynamics of treatment for patient-specific internal carotid aneurysms will be made by stents with different structures and different cross section of strut at the same porosity. In order to perform the study mentioned above, first patient-specific vessel model must be established based on medical scan images. Based on the idea of reverse engineering, patient-specific aneurysimal model was constructed. Numerical simulation of hemodynamics was performed to the aneurysimal model and the models with stent.Taken the calculation efficiency of Computational Fluid Dynamics and reducing calculation errors for patient-specific vascular model as evaluation index, advantages and disadvantages about two calculatin flows from medical images to numerical simulation were discussed (based on STL model and NURBS model respectively). Taken the construction of surface model of arteries fast and accurately as evaluation indexes, advantages and disadvantages about different methods of image segmentation were discussed. To improve the quality of surface mesh for arterial vessel model, specific methods of improving the quality of the surface mesh were discussed.In order to facilitate the study of biomechanics, a patient-specific thoracic aortic model and a patient-specific internal carotid aneurysm model were constructed respectively based on the method mentioned above. In accordance with the method to generate NURBS surface model used in reverse engineering, a vessel wall model of internal carotid artery was established. The established patient-specific model of internal carotid arterial aneurysm has not only a sidewall aneurysm but also a large S-shaped bend. Vessels with S-shaped bend are very common in the human vascular system and worthy of study using the established model. Pulsatile Fluid-Structure Interaction simulation was performed using the patient-specific internal carotid arterial model in order to study the spacial distribution characteristics and temporal changing characteristics of Von Mises Stress on the vascular wall. Static finite element analysis was carried out using the vessel wall model of internal carotid artery so as to determine whether Fluid-Structure Interaction simulation can be replaced by static analysis or not when studying distribution characteristics of Von Mises Stress. Although the geometry shape of patient-specific vessel model is very complex, comparason analysis showed that Fluid-Structure Interaction simulation can be replaced by static analysis when studying distribution characteristics of Von Mises Stress. Difference between them is small. A comparison study between the blood flow velocities obtained from clinical practice and those extracted from the result of the numerical simulation was performed, and a preliminary accuracy of the numerical simulation was verified in this paper.Five virtual stents with different structures and wire cross-sections were designed for incorporation into the same patient-specific aneurysm model in order to investigate the influence of stent structure on the hemodynamics of aneurysm. Computational fluid dynamics simulations were performed using ANSYS CFX11.0 so as to study how these five types of stent affect the hemodynamic parameters before and after intervention. Hemodynamic parameters such as streamline in aneurysimal cavity, velocity isosurface, wall shear stress and wall pressure were extracted and analysed for models with and without stent. Numerical results demonstrated that the mean flow rate in the aneurismal cavity decreased. Impact area of blood flow to aneurysimal distal wall reduced the most in the model that used a stent with a rectangular wire cross-section and spiral stent with a circular wire cross-section. Local high wall shear stresses at the dome of the aneurysm disappeared completely in models with stent. The wall pressure on the aneurysm increased slightly after implantation of the stent in all five models, but the increased value is small.The method mentioned above has the advantages of generality. It can be applied to the establishment of not only human arterial vessel but also other similar vessel and even patient-specific finite element model of other organs. Patient-specific model based on medical scan images is very close to the true vessel shape in geometry. It laid a solid foundation for reflection of the true blood flow (especially local flow characteristics). The main innovations of this paper are as follows: (1) Pulsatile Fluid-Structure Interaction numerical simulation was performed to the patient-specific internal carotid artery model with a large S-bend and a sidewall aneurysm. Comparison of the numerical simulation results showed that Fluid-Structure Interaction calculation can be replaced by static analysis when assessing magnitude and distribution of Von Mises Stress on aneurismal inner wall in numerical simulation of hemodynamics of aneurysm. (2) Five different stents were implanted into internal carotid model and their porosity is approximately same. Comparison studies about their effects to the hemodynamics parameters of aneurismal cavity were performed. The results demonstrated that stent with rectangular wire cross-section has a better performance when comprehensively considering the reduction of impact area to distal aneurismal wall, the reduction of WSS on aneurismal wall and bending ability of stent. These results can provide theoretical guidance for the design of endovascular stent. (3) It is found that aneurismal wall pressure increased slightly after stent implantation (comparison with the model without stent). This result is not consistent with published literature which stated that aneurismal wall stress decreases slightly after stent implantation. The reason for this difference is that the boundary conditions in the models are different. Comparing models with and without stent, the drop in pressure is set the same in some pulbished literature. However, in the present study, the inlet velocity is set to be the same for all the models. The blood flow conditions used in models with and without stent in some published literature are not the same. The blood flow velocity used in the model with stent is less than that used in the model without stent. After stent implantation, blood flow resistance increases. Therefore, with the same pressure drop, the inlet velocity would be reduced, and the blood flow used in model with stent would also be reduced. For the convenience of comparability, boundary conditions of equal blood flow should be used. Through the static structure finite element analysis to the mesh stent and spiral stent, it is found that flexility of mesh stent is better than spiral stent (including bending and twisting deformation capacity).