MRA of the Circle of Willis

These two photo galleries present the anatomy of Circle of Willis by means angio-MRI (Maximum Intensity Projection Time-Of-Flight).

Circle of Willis: AP view with rotations

Circle of Willis: lateral view with rotations

The circle of Willis plays a crucial part in maintaining blood flow in the brain. The circle of Willis is where several arteries in the brain meet or join together(1).

Also known as the circulus arteriosus cerebri or the cerebral arterial circle, the circle of Willis is an anastomotic (connecting) ring of arteries found at the base of the brain(2).

This arterial circle allows the two major arterial systems in the brain, the internal carotid arteries and vertebrobasilar systems (vertebral and basilar arteries), to connect with each other(3).

Functions of the Circle of Willis

The circle of Willis allows the delivery or flow of collateral blood between the anterior (front) and posterior (back) arterial systems of the brain.

The internal carotid arteries within the circle of Willis branch out and lead to smaller arteries(4). These arteries ensure that oxygenated blood is supplied to almost 80% of the cerebrum or upper region of the brain.

Moreover, the circle of Willis serves as a line of defense against ischemia (decrease in blood flow) in case of damaged or diseased blood vessels in one or more areas of the brain(5).

Diagnostic Imaging of the Circle of Willis

Studying the structure and anatomy of the circle of Willis used to require invasive conventional angiography(6). Today, various noninvasive neuroimaging techniques allow doctors to observe and record useful information about its anatomy.

Among these imaging techniques are magnetic resonance (MR) angiography, Doppler ultrasound, and computed tomography (CT) angiography(7).

Several noninvasive imaging modalities are often used in combination to study the circle of Willis, particularly MR angiography (MRA) and Doppler ultrasound(8).

Structure of the Circle of Willis

The circle of Willis surrounds different structures found within the interpeduncular fossa or the depression at the bottom of the brain(9).

The typical construction of the circle of Willis consists of three cerebral arteries (anterior, middle, and posterior) and two communicating arteries(10).

Communicating arteries are the blood vessels that connect the internal carotid arteries and the vertebrobasilar system(11).

The two major arterial systems that join together in the circle of Willis each supply blood to different areas of the brain(12).

The bilateral internal carotid arteries provide blood to most of the cerebral hemispheres(13). These areas include the parietal lobes, frontal lobes, lateral temporal lobes, and the deep cerebral hemispheres’ anterior or front part.

Meanwhile, the bilateral vertebral arteries ensure blood supply to the cerebellum, brainstem, occipital lobes, medial temporal lobes, the thalamus, and other areas at the rear of the deep cerebral hemisphere(14).

Anatomic Variations of the Circle of Willis

There are significant structural variations in the circle of Willis. In a study, researchers noted that a small part of the population possesses a complete circle of Willis(15)

Out of 300 study subjects who underwent MR angiograms over a period of two years, a complete circle of Willis was observed in 50 of the study subjects.

The study indicated a 16.6% prevalence of a complete cerebral arterial circle configuration(16). Additionally, the presence of a complete circle of Willis is slightly higher among women and subjects younger than 50 years.

The researchers also identified 10 types of variations each for the anterior and posterior part of the circle, based on the arrangement of the cerebral arteries(17)

The circle of Willis may also be categorized between symmetric or asymmetric variations(18)

A different study stated that the anomalies of the circle of Willis, such as duplications or triplications of the communicating arteries, may be associated with the likelihood of cerebrovascular disorders, stroke, and aneurysms(19).

Aneurysms and the Circle of Willis

The circle of Willis is a frequent location of intracranial aneurysms(20).

Aneurysms are weak and swelling areas in the arteries(21). Most aneurysms are asymptomatic until they rupture and require immediate treatment(22).

In a ruptured brain aneurysm, which causes a potentially fatal type of stroke known as subarachnoid hemorrhage (SAH), blood fills the cavity between the brain and skull(23).

Saccular aneurysms are the most common type of brain aneurysms. This type of aneurysm mostly occurs in and around the circle of Willis, particularly at the junctions of the blood vessels(24).

One study hypothesized that the symmetrical or asymmetrical configuration of the circle may be linked to a risk of cerebral aneurysm ruptures(25).

The researchers identified four basic types of configurations and one sub-type. The published findings suggested that an asymmetric circle of Willis may considerably raise the risk of brain aneurysm rupture(26).

Moreover, the researchers suggested using the circle of Willis configuration types as a basis to monitor for potential aneurysms using nuclear MR angiography or multi-slice CT scans(27).

An MRA, in particular, can help detect aneurysms in the circle of Willis. MRAs are able to depict the presence of vessel segments with at least 1mm diameter and are commonly used to diagnose intracranial aneurysms within the circle(28).

  1. Harnsberger HR, Osborn AG, Ross JS, Moore KR, Salzman KL, Carrasco CR, Halmiton BE, Davidson HC, Wiggins RH. Diagnostic and Surgical Imaging Anatomy: Brain, Head and Neck, Spine. 3rd ed. Salt Lake City, Utah. Amirsys. 2007.
  2. Bourjat P, Veillon F. Imagerie radiologique tête et cou. Paris, Vigot. 1995.
  3. Gouazé A, Baumann JA, Dhem A. Sobota. Atlas d’Anatomie humaine. Tome 3. Système nerveux central, système nerveux autonome, organe des sens et peau, vaisseaux et nerfs périphériques. 1er éd. Paris, Maloine. 1977.
  4. Kahle W, Cabrol C. Anatomie. Tome 3: Système nerveux et organe des sens. 1er éd. Paris, Flammarion. 1979.
  5. Ocean, E. (2020, Oct. 29). Circle of Willis. Kenhub. Retrieved from
  6. Ibid.
  7. Ibid.
  8. Medline Plus. (2020, June 23). Circle of Willis. Retrieved from,over%2080%25%20of%20the%20cerebrum
  9. Rosner, J., Reddy, V., & Lui, F. (2020, Jan.). Neuroanatomy, Circle of Willis. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Retrieved from
  10. Kalsoum, E., Leclerc, X., Drizenko, A., & Pruvo, J. P. (2014). Circle of Willis. Encyclopedia of the neurological sciences (second edition), 803-805.
  11. Ibid.
  12. Ibid.
  13. Ocean, E. (2020, Oct. 29). Op. cit.
  14. Ibid.
  15. Ibid.
  16. Rosner, J., Reddy, V., & Lui, F. (2020, Jan.). Op. cit.
  17. Ibid.
  18. Ibid.
  19. Naveen, S. R., Bhat, V., & Karthik, G. A. (2015). Magnetic resonance angiographic evaluation of circle of Willis: A morphologic study in a tertiary hospital set up. Annals of Indian Academy of Neurology, 18(4), 391–397.
  20. Ibid.
  21. Ibid.
  22. Stojanović, N. N., Kostić, A., Mitić, R., Berilažić, L., & Radisavljević, M. (2019). Association between Circle of Willis Configuration and Rupture of Cerebral Aneurysms. Medicina (Kaunas, Lithuania), 55(7), 338.
  23. Iqbal S. (2013). A comprehensive study of the anatomical variations of the circle of willis in adult human brains. Journal of clinical and diagnostic research : JCDR, 7(11), 2423–2427.
  24. Rosner, J., Reddy, V., & Lui, F. (2020, Jan.). Op. cit.
  25. Brain Aneurysm Foundation. (n.d.). Brain Aneurysm Basics. Retrieved from
  26. Harrison, G., & West, M. (2003). Subarachnoid Hemorrhage. Encyclopedia of the neurological sciences, 417-425.
  27. Ibid.
  28. Ibid.
  29. Stojanović, N. N., et al. (2019). Op. cit.
  30. Ibid.
  31. Ibid.
  32. Leclerc, X., Drizenko, A., & Pruvo, J. P. (2003). Circle of Willis. Encyclopedia of the neurological sciences, 725-728.
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