Impact of anatomical variations of the circle of Willis on the blood flow within unruptured intracranial aneurysm

Abstract

Background: Anatomical variations of the circle of Willis (CoW) are closely associated with the occurrence of intracranial aneurysms (IAs). However, the impact of anatomical variations on the rupture risk of IAs remains unclear. The purpose of this study was to explore the effect of artery absence on the internal flow and rupture risk of IAs.

Methods: A one-dimensional (1D) solver was combined with a 3-dimensional (3D) fluid-structure interaction (FSI) model to effectively quantify the hemodynamic characteristics inside IA under artery absence.

Results: The 1D results showed that the absence of anterior cerebral artery A1 segment (ACA-A1) or posterior cerebral artery P1 segment (PCA-P1) will trigger a compensatory blood flow effect, leading to significant blood flow variations of the anterior communicating artery (ACoA) and internal carotid artery (ICA). By FSI calculation, in the absence of ACA-A1, the maximum wall shear stress (WSS) within the ACoA aneurysm increased by 103% or more compared to the complete CoW due to blood inflow jet. In addition, WSS increased by 45% and 12% in the contralateral ICA and posterior communicating artery (PCoA) aneurysm respectively, whereas it decreased by 33% and 35% in the ipsilateral ICA and PCoA aneurysm, respectively. The absence of PCA-P1 had a less significant impact on the global blood flow of the CoW compared to the absence of ACA-A1, but it still led to an increase in WSS within the ipsilateral ICA and PCoA aneurysms (25% and 22%, respectively).

Conclusions: The absence of ACA-A1 or PCA-P1 may serve as an IA rupture risk factor. If ACA-A1 or PCA-P1 absence is identified clinically alongside an aneurysm, proactive treatment strategies are advised.

Keywords: Intracranial aneurysm (IA); anatomical variations; circle of Willis (CoW); fluid-structure interaction (FSI).

Figure 1

Schematic diagram of the circle of Willis.

Figure 2

Boundary conditions and grid independent verification for the ACoA aneurysm: (A) L-ACA-A1 inlet velocity waveform and L-ACA-A2 outlet pressure waveform during one cardiac cycle; (B) maximum WSS and wall pressure in the ACoA aneurysm under varying mesh densities. ACoA, anterior communicating artery; L-ACA-A1, left-anterior cerebral artery, A1 segment; L-ACA-A2, left-anterior cerebral artery, A2 segment; WSS, wall shear stress.

Figure 3

The vascular geometric models of the four IA patients: (A) ACoA aneurysm, (B) L-MCA aneurysm, (C) L-ICA aneurysm, (D) L-PCoA aneurysm. The arrow indicates the direction of blood flow entry. ACoA, anterior communicating artery; L-ICA, left-internal carotid artery; L-MCA, left-middle cerebral artery; L-PCoA, left-posterior communicating artery.

Figure 4

The average flow of the arteries in the CoW over one cardiac cycle (mL/min): (A) schematic diagram of the simulated arterial network in the CoW; (B) complete CoW; (C) L-ACA-A1 absence; (D) R-ACA-A1 absence; (E) L-PCA-P1 absence; and (F) R-PCA-P1 absence. The direction of the arrows indicates the average blood flow direction, and the dashed lines represent aneurysms. CoW, circle of Willis; L-ACA-A1, left-anterior cerebral artery, A1 segment; L-PCA-P1, left-posterior cerebral artery, P1 segment; R-ACA-A1, right-anterior cerebral artery, A1 segment; R-PCA-P1, right-posterior cerebral artery, P1 segment.

Figure 5

WSS and pressure distribution of ACoA aneurysm at peak systole. (A,G) Complete CoW; (B,H) L-ACA-A1 absence; (C,I) L-PCA-P1 absence. OSI distribution within one cardiac cycle. (D) Complete CoW; (E) L-ACA-A1 absence; (F) L-PCA-P1 absence. The red arrow indicates the size of the point parameter. ACoA, anterior communicating artery; CoW, circle of Willis; L-ACA-A1, left-anterior cerebral artery, A1 segment; L-PCA-P1, left-posterior cerebral artery, P1 segment; OSI, oscillatory shear index; R-ACA-A1, right-anterior cerebral artery, A1 segment; WSS, wall shear stress.

Figure 6

Von Mises stress distribution of four aneurysms at peak systole under complete CoW: (A) ACoA aneurysm; (B) L-MCA aneurysm; (C) L-ICA aneurysm; (D) L-PCoA aneurysm. ACoA, anterior communicating artery; CoW, circle of Willis; L-ICA, left-internal carotid artery; L-MCA, left-middle cerebral artery; L-PCoA, left-posterior communicating artery.