Circle of Willis
In this video, Dr. Bailey reviews the anatomy of the Circle of Willis, or the confluence of the internal cerebral and basilar arteries within the brain. She reviews the normal anatomy, talks about some common variants you may encounter, and introduces a few less common variants.
Introduction to the Circle of Willis
The Circle of Willis is the circular anatomical construct of vessels made up by the internal carotid arteries, the basilar artery, and their intracranial proximal branches. This includes the anterior and posterior communicating arteries and the anterior, middle, and posterior cerebral arteries.
The posterior circulation includes the posterior cerebral arteries, the basilar artery, the superior cerebellar artery, the anterior inferior cerebellar artery (AICA), and the posterior inferior cerebellar artery (PICA). The AICA is particularly variable and may be hard to see, particularly on 3D imaging. The PICA arises from the vertebral artery and may also vary in size from one side to the other.
One particularly common variant is a vertebral artery that terminates in PICA. That is, there is either no or a very vertebral artery is seen distal to the PICA origin. Dolichoectasia is a tortuous and prominent basilar artery larger than 4.5 mm in transverse diameter. It is also a common variant to have no posterior communicating arteries (P-comms). A fetal PCA, is vessel that arises from the posterior communicating artery with an absent or very small P1 segment of the PCA. A hypoplastic A1 is a small A1 on one side, with both A2 segments arising from one side. The A1 segments may arise at various levels and be tortuous. An azygous ACA is a single, or unpaired, ACA in the A2 segment where both sides fuse and there is a common ACA. Sometimes you can have the opposite and have 3 A2 segments. Any of the arteries can also be duplicated, or you can have a fenestration, a small wall within the center of the vessel. Fenestrations can mimic thrombus but they are often very linear along the course of the vessel.
Less common variants
The persistent trigeminal artery is a persistent fetal connection between the anterior and posterior circulation at the level of trigeminal artery. It is the most common persistent fetal connection and passes through Meckel’s cave (the trigeminal cistern).
It’s also possible to see a missing vessel, such as an absent ICA. In these cases, they may be congenitally absent or chronically occluded.
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