Flow Behavior and Distribution of Embolus-Model Particles at the Terminal Bifurcation of the Human Internal Carotid Artery

BACKGROUND: To investigate the possible role of fluid mechanical factors in thromboembolism that occurs at a high rate in the human middle cerebral artery (MCA). METHODS: Isolated transparent cerebral arterial trees containing the terminal bifurcation of the internal carotid artery (ICA), where the ICA bifurcated into the MCA and the anterior cerebral artery, were prepared from human cadavers. The flow behavior and distribution of embolus-model polystyrene particles in dilute suspensions at the bifurcation were studied in detail by means of flow visualization and high-speed cinemicrographic techniques. RESULTS: Large particles in suspensions flowing through the ICA migrated radially away from the vessel wall toward the axis of the ICA. It became more remarkable by increasing the flow rate in the ICA (Q(0)), flow rate ratio of Q(1) to Q(0) (MCA/ICA), and particle diameter. As a result, redistribution of particles flowing in the ICA occurred at the bifurcation. The particles >1 mm in diameter (approximate to 1/4 of vessel diameter) selectively entered the MCA, even when the flow rate ratio of Q(1) to Q(0) was decreased to as low as 0.34. In contrast, the particles whose diameters were <0.3 mm (approximate to 1/10 of vessel diameter) and that did not show radial migration entered the MCA at the same rate as the flow rate ratio of Q(1) to Q(0). CONCLUSIONS: Due to the flow-dependent migration of particles away from the vessel wall toward the axis of the ICA, large particles selectively enter the MCA to which the core flow of the ICA is generally directed. This might explain why the incidence of thromboembolism is higher in the MCA than in the anterior cerebral artery in humans.