Complex flow in the CB. Ultrasound vector flow imaging shows changes of direction and velocity of the flow layers due to vessel dilatation, resulting in the development of complex flow in both the internal carotid artery (ICA) and external carotid artery (ECA). High‐velocity red vectors flowing near the flow divider in the ICA (arrow) and a wide area of multidirectional low‐velocity green vectors (asterisk) are shown in the ICA sinus. Wall shear stress measurements on the outer wall (blue dots) of the ICA gave normal maximum values (1.0–1.6 Pa) in all of the samples and abnormal mean values (0.1–0.2 Pa) in 2 of the 3 sample points. CCA indicates common carotid artery.