Background and Purpose The aim of this study was to assess the anatomy of the circle of Willis in preterm neonates (gestational age 25–31 weeks) at term equivalent age and to evaluate the relation between anatomical variations and blood flow through the internal carotid artery and basilar artery.

Methods Flow measurements (ml/min) were obtained with two-dimensional phase-contrast magnetic resonance angiography (MRA). Time-of-flight MRA was used to assess the anatomy of the circle of Willis for a dominant A1 segment of the anterior cerebral artery or a fetal-type posterior cerebral artery. Differences in flow were assessed with analysis of variance.

Results In our cohort, 74% showed a variant type of the anatomy of the circle of Willis. The flow in the internal carotid artery at the side of a dominant A1 segment (43.3 ml/min) was increased compared with both the flow in the contralateral internal carotid artery (33.0 ml/min) and the flow in the internal carotid artery in children with a normal anterior anatomy (38.4 ml/min). The flow in the basilar artery was highest in neonates with a normal configuration of the posterior part of the circle of Willis (32.6 ml/min) compared with children with a unilateral 25.3 (ml/min) or bilateral fetal-type posterior cerebral artery (18.6 ml/min).

Conclusion Preterm neonates show a high prevalence of variant types of the circle of Willis at term equivalent. We demonstrated the relation between variations in the circle of Willis and volume flow measurements in the internal carotid artery and basilar artery.