Distribution of cerebral blood flow in the circle of Willis

PURPOSE: To prospectively determine the effect of anatomic variations in the circle of Willis on volume flow in the internal carotid arteries (ICAs) and basilar artery (BA). MATERIALS AND METHODS: Institutional review board approval and informed consent were obtained. Phase-contrast magnetic resonance (MR) angiography was used to measure the volume flow in the BA and ICAs in 208 patients (182 men, 26 women; mean age, 60 years) with symptomatic atherosclerosis or risk factors for atherosclerosis. Patients with steno-occlusive disease were excluded, and flow values were normalized for age. Three-dimensional time-of-flight MR angiograms were used to assess the anatomy of the circle of Willis. Differences in volume flow between a complete circle of Willis, a circle with a missing A1 segment, and a circle with a fetal-type posterior cerebral artery were analyzed (analysis of variance and scheffe post hoc tests). RESULTS: The ICA volume flow in subjects with a complete configuration of the circle of Willis was 245 mL/min +/- 65 (standard deviation). Flow in the contralateral ICA was significantly increased (P <.01) in subjects with a missing All segment (303 mL/min 56) compared with control subjects and compared with flow on the ipsilateral side (214 mL/min 94; P <.01). In subjects with a unilateral or bilateral fetal-type posterior cerebral artery, the ICA volume flow was increased (P <.01) and the BA volume flow was decreased (P <.01) in comparison with the flow in subjects with no fetal-type circle of Willis. CONCLUSION: Large asymmetries in volume flow between the right and left ICAs or decreased volume flow in the BA is not necessarily caused by vascular disease but may be caused by variations in the anatomy of the circle of Willis.