The circle of Willis encircles the stalk of the pituitary gland and provides important communications between the blood supply of the forebrain and hindbrain (ie, between the internal carotid and vertebro-basilar systems following obliteration of primitive embryonic connections).
Although a complete circle of Willis is present in some individuals, it is rarely seen radiographically in its entirety; anatomical variations are very common and a well-developed communication between each of its parts is identified in less than half of the population.
The circle of Willis begins to form when the right and left internal carotid artery (ICA) enters the cranial cavity and each one divides into two main branches: the anterior cerebral artery (ACA) and middle cerebral artery (MCA).
The anterior cerebral arteries are then united and blood can cross flow by the anterior communicating (ACOM) artery. The ACAs supply most midline portions of the frontal lobes and superior medial parietal lobes. The MCAs supply most of the lateral surface of the hemisphere, except the superior portion of the parietal lobe (via ACA) and the inferior portion of the temporal lobe and occipital lobe. The ACAs, ACOM, and MCAs form the anterior half, better known as the anterior cerebral circulation. Posteriorly, the basilar artery (BA), formed by the left and right vertebral arteries, branches into a left and right posterior cerebral artery (PCA), forming the posterior circulation.
The PCAs mostly supply blood to the occipital lobe and inferior portion of the temporal lobe.
The PCAs complete the circle of Willis by joining the anterior circulation formed by the ICAs via the posterior communicating (PCOM) arteries. See the images below.
Schematic of the circle of Willis and cerebral vasculature in relation to local anatomy. Courtesy of Wikimedia Commons.
Magnetic resonance arteriography illustrating the circle of Willis and its branches.