Understanding of boundary conditions imposed at multiple outlets in computational haemodynamic analysis of cerebral aneurysm

Computational fluid dynamics (CFD) is considered to be a promising tool for haemodynamic analysis of the intracranial aneurysm. However, aneurysm CFD is still not regarded as fully reliable mainly because the computational result is influenced by too many factors such as the luminal geometry of the model, spatiotemporal resolutions and boundary conditions. Among the influential factors, this paper focuses on outflow boundary conditions used when the computational domain has multiple outlets. Four outflow strategies found in published articles are reviewed: 1) pre-scription of constant or zero pressure, 2) flow splitting based on the power law, 3) traction-free and zero velocity-gradient conditions and 4) coupling of CFD with a reduced-order model. None of them has proved definitely superior or inferior to others. For accurate quantification of the haemodynamic state in the aneurysm, it is crucial to incorporate the physiologically correct flow splitting ratio in CFD analysis by means of accurate specification of pressure or flow rate at the outlets. A coupling of CFD and a 0-d model (a subtype of the reduced-order model) appears to be the most promising although further study is necessary to achieve accurate estimation of model parameters.