<P> Intracranial aneurysm, also known as brain aneurysm, is a cerebrovascular disorder in which weakness in the wall of a cerebral artery or vein causes a localized dilation or ballooning of the blood vessel . </P> <P> Aneurysms in the posterior circulation (basilar artery, vertebral arteries and posterior communicating artery) have a higher risk of rupture . Basilar artery aneurysms represent only 3% - 5% of all intracranial aneurysms but are the most common aneurysms in the posterior circulation . </P> <P> Cerebral aneurysms are classified both by size and shape . Small aneurysms have a diameter of less than 15 mm . Larger aneurysms include those classified as large (15 to 25 mm), giant (25 to 50 mm), and super-giant (over 50 mm). </P> <P> Saccular aneurysms, also known as berry aneurysms, appear as a round outpouching and are the most common form of cerebral aneurysm . </P>

Most common site of aneurysm in circle of willis