Haemodynamic characteristics of thin-walled regions in intracranial aneurysms: intraoperative imaging and CFD analysis.

Background: Identifying haemodynamic factors associated with thin-walled regions (TWRs) of intracranial aneurysms is critical for improving pre-surgical rupture risk assessment. Intraoperatively, these regions are visually distinguished by a red, translucent appearance and are considered highly rupture prone. However, current imaging modalities lack the resolution to detect such vulnerable areas preoperatively. This study aimed to determine whether thin-walled regions exhibit distinct local haemodynamic profiles compared to adjacent normal-appearing wall regions.

Methods: Sixteen patient-specific models of unruptured middle cerebral artery aneurysms were reconstructed from digital subtraction angiography images. Intraoperative TWRs were identified using a colour segmentation method based on Delta E metrics. Computational fluid dynamics (CFD) simulations were used to compute six haemodynamic parameters: wall shear stress (WSS), time-averaged WSS (TaWSS), oscillatory shear index (OSI), relative residence time (RRT), WSS divergence (WSSD), and pressure. Haemodynamic data were extracted from spatially localised surface patches within confirmed thin and normal regions. Linear mixed-effects models were applied to compare parameters while accounting for patient-level and intra-patient variability, using normalised values to improve model fit.

Results: Thin regions exhibited significantly higher WSS, TaWSS, WSSD, and pressure, and reduced RRT. WSS and TaWSS were approximately 3.3% and 2.8% higher in TWRs, respectively. WSSD was 5.4% higher and RRT was 0.3% lower, suggesting faster, more divergent flow in thin regions. Pressure was modestly but significantly elevated at + 1.3%. No significant difference was observed in OSI between regions.

Conclusions: Thin-walled regions in intracranial aneurysms demonstrate a distinctive haemodynamic profile characterised by stronger, sustained shear forces, greater shear divergence, and reduced residence time, suggesting a dynamic mechanical environment that promotes focal wall thinning. Our findings suggest that persistent shear-driven stress, rather than oscillatory flow, is a key haemodynamic feature of thin-walled regions and may contribute to localised aneurysm wall vulnerability.