The CT angiography shows the circle of Willis (CW), which is the primary collateral system of the patients with steno-occlusive disease of the carotid. The red arrow signs the right internal carotid artery. It branches the medial cerebral artery (two red asterisks), the anterior cerebral arteries and the anterior communicant artery (three red asterisks), which constitute the anterior circle of CW. The one red asterisk signs the basilar artery, which branches the posterior cerebral artery and the posterior communicant arteries creating the posterior circle of CW