Zeng, Zijing

Hemodynamic factors are thought to play an important role in the initiation, growth, and rupture of cerebral aneurysms. In-vitro studies have demonstrated a correlation between the magnitude and distribution of wall shear stress (WSS) and biological response of both endothelial cells and smooth muscle cells [1–3]. In elastase induced saccular aneurysms, low WSS (below 0.5 Pa) was found to have a correlation with altered expression of biological markers [4]. Localized regions of rapid aneurysm growth in-vivo have been shown to be associated with regions where WSS is below a critical value of 0.1 Pa [5]. Further, aspect ratio (AR), the ratio of the maximum diameter of the aneurysm to the width of the aneurysm neck, has been correlated with elevated risk of rupture [6]. The purpose of the current study is to explore the possibility of creating elastase induced aneurysms in rabbits with a range of aspect ratios (ratio of aneurysm height/neck) and evaluate the existence of a correlation between aspect ratio and WSS distribution. Aneurysms with ARs from 0.98 to 2.8 were created at the origin of the right common carotid artery (n = 30). Qualitative differences in WSS distribution were found in the high AR aneurysms (HARA) (AR>1.6) and low AR aneurysms (LARA) (AR<1.6) [7].

An intracranial aneurysm (IA) is a pathological condition of cerebral arteries characterized by local enlargements of the arterial wall, typically into a saccular shape. Rupture of the aneurysm sac can result in devastating cerebral hemorrhage. Hemodynamic factors are believed to play an important role in initiation, development and rupture of IAs [1–3]. However, the coupling between hemodynamics and aneurysm pathophysiology is complex and remains poorly understood. Patient specific diagnostics regarding risk of rupture can be substantially advanced by improving our understanding of the in-vivo response of the aneurysm wall to intra-saccular hemodynamic stresses. A mechanism for fundamental studies of the impact of chronically altered WSS on the intact vascular wall is provided by animal models. However, cerebral aneurysms have not been shown to occur naturally in animals. Thus, a number of animal models have been created for studying aneurysm pathogenesis including those in mice, rats, rabbits, canines, swine and primates. To make meaningful use of these models, it is important to evaluate their relevance to human biomechanics and pathophysiology.

Computational fluid dynamics (CFD) studies provide a valuable tool for evaluating the role of hemodynamics in vascular diseases such as cerebral aneurysms and atherosclerosis. However, such models necessarily only include isolated segments of the vasculature. In this work, we evaluate the influence of geometric approximations in vascular anatomy on hemodynamics in elastase induced saccular aneurysms in rabbits. One representative high aspect ratio (AR—height/neck width) aneurysm and one low AR aneurysm were created at the origin of the right common carotid artery in two New Zealand white rabbits. Three-dimensional (3D) reconstructions of the aneurysm and surrounding arteries were created using 3D rotational angiographic data. Five models with varying extents of neighboring vasculature were created for both the high and low AR cases. A reference model included the aneurysm sac, left common carotid artery (LCCA), aortic arch, and downstream trifurcation/quadrification. Three-dimensional, pulsatile CFD studies were performed and streamlines, wall shear stress (WSS), oscillatory shear index, and cross sectional velocity were compared between the models. The influence of the vascular domain on intra-aneurysmal hemodynamics varied between the low and high AR cases. For the high AR case, even a simple model including only the aneurysm, a small section of neighboring vasculature, and simple extensions captured the main features of the steamline and WSS distribution predicted by the reference model. However, the WSS distribution in the low AR case was more strongly influenced by the extent of vasculature. In particular, it was necessary to include the downstream quadrification and upstream LCCA to obtain good predictions of WSS. The findings in this work demonstrate the accuracy of CFD results can be compromised if insufficient neighboring vessels are included in studies of hemodynamics in elastase induced rabbit aneurysms. Consideration of aspect ratio, hemodynamic parameters of interest, and acceptable magnitude of error when selecting the vascular domain will increase reliability of the results while decreasing computational time.