Temporal variations of wall shear stress parameters in intracranial aneurysms-importance of patient-specific inflow waveforms for CFD calculations

Purpose: To assess reliability of wall shear stress (WSS) calculations using computational fluid dynamics (CFD) dependent on inflow in internal carotid artery aneurysms (ICA). Materials and methods: Six unruptured ICA aneurysms were studied. 3D computational meshes were created from 3D digital subtraction angiographic images (Axiom Artis dBA, Siemens Medical Solutions). Transient CFD simulations (Fluent, ANSYS Inc.) were performed for two inflow conditions: (1) idealized averaged waveform from normal subjects (ID) and (2) patient-specific waveform (PS) measured with 2D phase contrast magnetic resonance imaging. Stability of calculation was assessed by comparing mean WSS (<WSS>), temporal wall shear stress magnitude variation (ΔWSS), and oscillatory shear index (OSI, a measure of variation in the WSS direction) on the aneurysmal wall for both conditions. Results: For all cases, mean relative difference (PS-ID) of WSS (<WSS>) was -15% (range -32% to 11%). Mean ΔWSS difference was -29.3% ( -100% to 67%). Mean OSI difference was 7.5% (-12% to 40%). Large variations in histograms of these parameters were noted. Conclusion: For accurate calculations of WSS parameters, patient-specific information on physiological flow may be necessary. Results obtained with averaged or idealized flow waveforms may have to be interpreted with caution.