Rupture status investigation of patient specific cerebral aneurysms by analysing hemodynamic factors using computational fluid dynamics

Cerebral aneurysms are rather common considering the worldwide scale of population and associated with high morbidity and mortality rate. In order to prevent disability problems and deaths caused by cerebral aneurysms, the researchers from both medical side and engineering side have been performing intensive studies. Although the intensive researches, cerebral aneurysms, particularly hemodynamic effects on cerebral aneurysms are not quite well understood. Hence, as long as the researches proceed, the studies in terms of aneurysm status might enlighten the mystery of hemodynamic effects on the cerebral aneurysms. Essentially, the aneurysms are pathologic and abnormal structures of the blood vessel walls. Cerebral aneurysms arise from the weakness of the cerebral artery caused by various factors such as genetics and hypertension. These structures are abnormal dilations of the arterial wall that mostly localized in the Circle of Willis (CoW). There are three phases for the cerebral aneurysms: formation, growth and eventually rupture. The rupture of the cerebral aneurysm might lead to serious disability problems for the patient or even worse, might be end up with the death of the patient. When the rupture is occured within the cerebral aneurysm, it might lead to the bleeding into the subarachnoid space in the brain. This phenomenon is called subarachnoid hemorrhage (SAH) and SAH is occured by cerebral aneurysms with a remarkably rate of 85%. As a consequence, it is significantly crucial to investigate rupture status of the cerebral aneurysms in order to predict the rupture risk. For a long time, over the centuries, circulatory system had been examined in behalf of discovering the organisms. Blood vessel remodeling with the flow conditions had been studied a long time ago in 1800s by Virchow. Thenceforward, the blood vessels and blood flow studies have been extensively performed in order to examine circulatory system. Before the computational-based investigations in the sense of the cerebral aneurysms and blood flow in arteries, animal-based experiments had been performed considerably to explore this secret world of the blood flow complexity in circulatory and cardiovascular system. The development of technology gives computer-based studies an opportunity in order to examine blood flow simulations that is unfeasible to perform by means of in-vivo methods. Particularly, with the increasing techniques and technologies with regard to imaging, it has become possible to perform computational fluid dynamics (CFD) within patient-specific cerebral aneurysms instead of performing simulations within the artificial straight pipe geometries that are not realistic and reliable. It has been demonstrated that the examination based on CFD and hemodynamic factors play a significant role in order to investigate cerebral aneurysm steps which are initiation, growth and rupture, respectively. However, there are still contradictory studies with regards to the rupture risk examinations of the cerebral aneurysms under hemodynamic factors and these factors are not completely understood. For this purpose, within this study, the rupture status of the patient-specific cerebral aneurysms have been examined with regards to hemodynamic factors by means of CFD. The CFD simulations have been performed by means of an open source CFD software based on C++ which is OpenFOAM (Open-source Field Operation And Manipulation). CFD techniques have been used extensively by researchers and become inspiring methods in terms of analyzing the effects of the hemodynamic factors on the image-based patient-specific cerebral aneurysms. However, the simulation results might be misleading if the grid resolution is not properly fit in with the abnormal and complex aneurysm geometry. In order to overcome this issue, there is a need for estimating the discretization errors in CFD studies. The error analysis studies have been performed extensively in CFD researches for decades, therefore, also within this thesis, grid convergence index (GCI) method based on generalized Richardson Extrapolation has been performed in order to estimate the discretization errors. Due to its non-Newtonian shear-thinning viscoelastic nature and wave-like Womersley velocity profile, mathematical modeling of blood flow is a challenging phenomenon, more particularly in complex and abnormal patient-specific vessels such as cerebral aneurysms. Moreover, the aneurysm sac region is an abnormal area and within this area blood flow tends to exhibit from transient to turbulence nature with the increasing numbers of Reynolds. Therefore, within this study, under the Navier-Stokes equations, the blood has been modeled as unsteady, incompressible, pulsatile flow with its non-Newtonian shear-thinning characteristic by taking the advantage of Carreau-Yasuda viscosity model. Regarding to the turbulence nature within the aneurysm sac, it has been examined turbulence effects by means of dynamic k-equation for large eddy simulations (LES). Regarding to the patient-specific cerebral aneurysm geometries, due to the lack of the vascular wall information in the sense of thickness and elasticity in the Aneurisk dataset repository, the vascular walls have been modeled as rigid with no-slip boundary conditions. All the simulations have been performed with the pisoFoam solver based on PISO (Pressure-Implicit with Splitting of Operators) algorithm under finite volume method (FVM) by using OpenFOAM. The original patient-specific cerebral aneurysm data have been clipped in order to examine the region of interests, added flow extensions in order to have proper inflow and outflow boundaries by using the Vascular Modeling Toolkit (VMTK), an open source software. In order to perform CFD simulations, triangular surface meshes and tetrahedral volume meshes have been generated by using ICEM CFD 19.2 (ANSYS, Inc., Canonsburg, PA, USA). The simulation results obtained have been visualized in terms of hemodynamic factors such as WSS, oscillatory shear index (OSI), time-averaged WSS (TAWSS), spatial WSS gradient (SWSSG) and time-averaged SWSSG (TASWSSG) by using ParaView, an open source visualization software. This thesis has been written by using LATEX. Within this study, the rupture status of one ruptured which belongs to a female patient aged 26 and one unruptured which belongs to a female patient aged 48, patient-specific cerebral aneurysm data from the Aneurisk dataset repository have been investigated in terms of hemodynamic factors by using CFD in order to observe the similarities and differences of rupture status with non-Newtonian shear-thinning characteristic of blood flow by implementing Carreau-Yasuda viscosity model with the pulsatile nature of blood flow. Moreover, it has been investigated turbulent flow nature of blood with dynamic k-equation for LES turbulence model in order to observe the non-Newtonian characteristic with turbulence effects, particularly in the aneurysm sac region. Within both ruptured and unruptured cerebral aneurysm data, the streamlines exhibit smoothly in the inlet area and tend to exhibit transient behavior within the curvature regions. The streamlines behave chaotic and exhibit turbulent nature when the flow arrives in the aneurysm sac area. The simulation results demonstrated that within the patient specific cerebral aneurysm data, particularly in the aneurysm sac, with the increasing Reynolds numbers especially at the peak systole, the mild turbulence effects have been observed. The WSS vectors exhibit different directions under the turbulence effects with the turbulence blood flow model, and as a consequence, OSI values increase. From the simulations results, considering the WSS values for both non-Newtonian Carreau-Yasuda (CY) and non-Newtonian CY with LES turbulence models, lower WSS has been observed for aneurysm sac region than parent vessel within ruptured cerebral aneurysm R − ICA while higher WSS has been observed for aneurysm sac region than parent vessel within unruptured cerebral aneurysm U − ICA. These results have similarities with other studies in the literature (e.g. Shojima et al.). In opposition to the literature, within this study, OSI values have been observed higher for the unruptured cerebral aneurysm U − ICA than the ruptured cerebral aneurysm R − ICA. In a major of the studies, high OSI values are associated with the ruptured cerebral aneurysms. Moreover, high OSI values are associated with endothelial cell dysfunctions and vascular remodeling of the cerebral arteries. As a consequence, since unruptured cerebral aneurysm U − ICA has high OSI values, it could be interpreted that there might be a high rupture risk within U − ICA geometry. However, in order to examine rupture status within cerebral aneurysms, there is a strong need for further series of analysis by researchers around the world. Moreover, since low WSS and high SWSSG are associated with disturbed flow, it might be interpreted that, ruptured cerebral aneurysm R − ICA has more disturbed flow regions than unruptured cerebral aneurysm U − ICA.