Fluid-structure interaction study of cerebral aneurysms

Abstract

A cerebral aneurysm (CA), or intracranial aneurysm (IA), is a pathological dilatation of the cerebral artery arising from the deterioration of internal elastic lamina (IEL) and media of the arterial wall which can lead to high mortality rate upon rupture. The underlying mechanism of the formation, growth, and rupture processes of CA is still not well understood. While numerical simulations can be an effective stool to get more insights in the hemodynamics and wall responses of the CAs. In this thesis, firstly the performance between computational fluid dynamics (CFD) and fluid-structure interaction (FSI) in CA analyses are compared. Results show that CFD overestimates the wall shear stress (WSS) than FSI. Secondly, the comparison between FSI numerical simulation and in vitro experiment is carried out. Results show that FSI analysis can give reliable results of the wall displacement. Thirdly, the comparative study of FSI performance between ADINA and ANSYS is investigated. Results suggest that ADINA may be a better choice in FSI analysis of CA. The effects of the blood viscosity, elastic modulus and hypertension on the hemodynamics and wall responses are investigated by carrying out FSI analyses on four patient-specific CAs. When considering the blood as a Newtonian fluid, changing the blood viscosity has a large effect on the WSS, while the effects on the flow pattern, the wall stresses and displacements are minimal. The current study also finds that the WSS predicted by non-Newtonian blood assumption has a great difference from that for Newtonian blood assumption. When considering the walls as linearly elastic, the maximum WSS at the control points and wall stresses at the dome will increase as the elastic modulus (E) increases from 1 to 20 MPa and they will become practically constant for E larger than 20 MPa. Besides, the differences of the wall stresses between hyperelastic and its equivalent linearly elastic assumptions are found to be as high as 35%. Hypertension increases the wall stresses and displacements and decreases the WSS at the control points, compared to the normal blood pressure. A scaling method is proposed to mimic the aneurysm growth process in which the aspect ratio (AR), being the ratio of the height to neck width, increases and the wall thickness (tw) decreases. Comparison of the FSI analyses in patient-specific CAs between before and after scaling shows that the wall stresses at the aneurysm dome where rupture often occurs will increase notably after scaling and the WSS inside the aneurysm will notably reduce after scaling.