The circle of Willis is an important junction of arteries at the base of the brain. The major extracranial arteries which feed the circle of Willis are the bilateral internal carotid arteries and the bilateral vertebral arteries.
The structure of the circle of Willis includes:
Connecting arteries to the circle of Willis include:
The circle of Willis plays a critical role in ensuring the brain receives adequate blood supply. It allows blood to flow from the anterior and posterior hemispheres, as well as from the left and right hemispheres of the brain. In other words, without the circle of Willis blood could not travel across the brain.
The circle of Willis is the most important source of collateral circulation in the presence of significant extracranial disease like stenosis, hemorrhage, or stroke. Collateral circulation provides alternative routes for blood flow when another artery becomes blocked or damaged. The natural structure of the circle allows for bi-directional blood flow in the event of significant changes in pressure in any brain segment. This means blood flow can actually change direction when necessary. These collateral pathways can assure adequate blood supply to all areas of the brain, reducing brain damage or other negative after effects.
Some common structural variations include:
Additional variations include:
Transcranial Doppler (TCD) is a non-invasive, painless ultrasound technique that utilizes high-frequency ultrasound waves to measure the rate and direction of blood flow inside the intracranial blood vessels. The test examines and records the speed of blood flowing in the circle of Willis to facilitate the diagnosis of a wide range of conditions.