<P> The redundancies that the circle of Willis introduce can also lead to reduced cerebral perfusion . In subclavian steal syndrome, blood is "stolen" from the circle of Willis to preserve blood flow to the upper limb . Subclavian steal syndrome results from a proximal stenosis (narrowing) of the subclavian artery, an artery supplied by the aorta which is also the same blood vessel that eventually feeds the circle of Willis via the vertebral artery . </P> <Ul> <Li> <P> Fetal ultrasound image at the level of circle of Willis, showing PCA, MCA and ACA </P> </Li> <Li> <P> Cerebral angiogram showing an anterior / posterior projection of the vertebro basilar and posterior cerebral circulation, the posterior aspect of the circle of Willis and one of its feeding vessels . </P> </Li> <Li> <P> An anterior view of major cerebral and cerebellar arteries . </P> </Li> <Li> </Li> <Li> <P> Circle of Willis </P> </Li> <Li> <P> Circle of Willis </P> </Li> </Ul> <Li> <P> Fetal ultrasound image at the level of circle of Willis, showing PCA, MCA and ACA </P> </Li> <P> Fetal ultrasound image at the level of circle of Willis, showing PCA, MCA and ACA </P>

Circle of willis is in the subarachnoid space