Circle of Willis

Circle of Willis

The Circle of Willis, long considered to be an important anatomic vascular formation, provides backup circulation to the brain. It connects the circulation in the right and left hemispheres with the circulation in the anterior and posterior hemispheres. Located at the base of the brain, a complete circle of Willis is the most important source of collateral circulation in the presence of extracranial carotid or vertebral artery disease. In case one of the supply arteries is occluded, the Circle of Willis provides interconnections between the internal carotid arteries and basilar artery along the floor of the cerebral vault, providing blood to tissues that would otherwise become ischemic.

Origin of Vessels

The left and right internal carotid arteries arise from the right and left common carotid arteries.

The posterior communicating artery is given off as a branch of the internal carotid artery just before it divides into its terminal branches – the anterior and middle cerebral arteries.

The anterior cerebral artery forms the anterolateral portion of the Circle of Willis, while the middle cerebral artery does not contribute to the circle.

The anterior communicating artery connects the two anterior cerebral arteries.

The right and left posterior cerebral arteries arise from the basilar artery, which is formed by the left and right vertebral arteries.

The vertebral arteries arise from the subclavian arteries.

Assessment of flow

Transcranial Doppler sonography is a noninvasive technique that uses a 2-MHz, pulsed Doppler transducer to measure the velocity of blood flow within the circle of Willis and vertebrobasilar system. Diagnoses made with transcranial Doppler sonography are based on the detection of increased or decreased blood flow velocity, absence of blood flow, or changes in pulsatility.

Transcranial Doppler examinations are performed through the following four, naturally occurring cranial windows (in addition to patent fontanebles or burr holes):

• Transtemporal
• Transorbital
• Transforaminal (foramen magnum)
• Submandibular

Three transtemporal windows are available due to regions of thinning in the suprazygomatic portion of the temporal bone. The posterior window lies immediately anterior to the external auditory canal, is available in most patients, and provides the best spatial separation of the anterior and posterior circulations. The middle window lies 1 .5 cm anterior to the posterior window, and the anterior window lies 1 .5 cm anterior to the middle window. All three windows should be routinely insonated to determine the optimal site for vessel localization in a particular patient. Rarely, a frontal window, just anterior to the pterion, can be used in place of the transtemporal approaches. The transtemporal windows are routinely used to study the middie, anterior, and posterior cerebral arteries, along with the anterior and posterior communicating arteries and the terminal portion of the internal carotid artery. The anterior and posterior communicating arteries are detectable by means of transcranial doppler only when they serve as collateral routes of circulation; that is, when they exhibit accelerated blood flow.

The transorbital window is available due to the thinness of the orbital plates of the frontal bones and bony defects caused by the optic foramina and superior orbital fissures. This location is used to study the ophthalmic artery and the three segments of the siphon of the internal carotid artery.

The transforaminal window is accessible when the patient’s head is flexed mildly forward and allows assessment of the intracranial portions of the basilar and vertebral arteries.

The submandibular window is used by placing the transducer in a superior direction just below the angle of the mandible; it allows study of the extradural segment of the internal carotid artery.

Last Modified: 10/13/2010
Created: 08/21/2010