Aneurysm Formation and Rupture in the Circle of Willis

Subarachnoid haemorrhage is the type of stroke that is associated with the high- est mortality (50%) and morbidity (40%). It is usu- ally caused by the rupture of an cerebral aneurysm, a balloon-like bulge of a vessel (see Figure 1). Around 1–6% of the general population develop aneurysms dur- ing a lifetime and often at quite an early age (mean age ≈ 50 years). Although aneurysms are relatively common, most of them are stable. The average annual risk of rupture is less than 1% and may therefore often be outweighed by the risk associated with treatment. The low risk, but deadly consequences of a rupture calls for patient-spesific risk assessment and has motived the computational community to perform numerical analysis of the problem.

Large retrospective studies demonstrate that computational fluid dynamics (CFD) can discriminate between ruptured and non-ruptured aneurysms more accurately than current clinical indicators. Even the ’AHA/ASA Guidelines for Management of Aneurysmal Subarachnoid Hemorrhage’ now recommends clinicians to ‘consider hemodynamic characteristics of the aneurysm when discussing the risk of aneurysm rupture’. However, patient-specific risk estimation cannot yet be accomplished with currently available modalities and indicators.

At Simula, we aim to improve the computational models such that they can be employed for patient-specific risk assessment. In particular, the current focus is to establish to what extent transition to turbulence, (see movie), occur in cerebral aneurysms and whether there is a close correlation between rupture risk and transition. Furthermore, we are investigating the non-linear rheology of blood and the fluid-structure interaction between the blood and vessels.