Hemodynamic factors play an important role in the initiation, progression and rupturing of intracranial aneurysms. In this study, Computational Fluid Dynamics (CFD) is used to investigate the effects of blood rheology models (Newtonian and Casson) on the hemodynamic parameters. The content of this research is divided into two parts and qualitative and quantitative results are presented. In the first part, effects of variations of the blood flow on aneurysmal hemodynamic in a ruptured internal carotid artery aneurysm are discussed for Newtonian and Casson model. CFD simulation was carried out for three situations: normal and 20% increase and decrease in the inflow. Qualitative observations indicate that: 1) increase in the inflow increases the difference between the Newtonian WSS (Wall Shear Stress) and Casson WSS. Quantitativ observations indicate that:WSS is more sensitive to the rheological models used for predicting flow behavior in aneurysms than velocity. In the second part, CFD simulations were carried out for ten saccular aneurysms to investigate the differences between the two models in different conditions. Qualitative observations indicate that: 1) WSS, Oscillatory Shear Index (OSI), velocity distribution and flow pattern have similar characteristics for both rheology models. 2) Newtonian model overestimates the velocity in the most areas, near the wall of aneurysm sac and parent vessel. 3) There is no preference in using Casson on Newtonian models for regions of low WSS. The same fact was observed for OSI and velocity. Quantitative observations indicate that: 1) Newtonian model overestimates the space-average velocity at peak systole in the aneurysm sac (about 12.5%). 2) The difference between (Casson and Newtonian) WSS predictions increases by decreasing the WSS values and increasing OSI over aneurysm wall. So, while studying near wall regions with certain biological mechanisms (e.g. thrombogensis) in mind, it is important to choose Casson model that appropriately model the shear-thining nature of blood. Otherewise, it is preferred choosing Newtonian model because of its ease of application. Keyword : Interacranial aneurysm, Non-Newtonian rheology, Computational fluid dynamics (CFD), Blood flow
