A Review of the Circle of Willis: Investigative Methods, Anatomical Variations and Correlated Ischemic Brain Diseases

The Circle of Willis (CoW) is the most important collateral pathway communicating between the bilateral carotid system and the posterior circulation. Many techniques have been used to investigate the configuration and function of the CoW and their advantages and disadvantages are reviewed here. In previous studies, morphometric variation in CoW has been widely detected; however, the frequency of variation ranges largely, between 10% and 85%. Some differences in reported frequency may reflect differences in: the definition of CoW variation, the methods used to examine variation and study populations. Two conclusions, however, are clear: (i) the prevalence of variation in the posterior circle is higher than in that of the anterior circle; and (ii) variations in the CoW are correlated with cerebral or carotid vessel diseases. Although the cause of the variants remains unclear, both genetic background and hemodynamics could play important roles. Lastly, the correlation between the CoW and ischemic brain diseases is discussed. In future, artificial intelligence will be helpful for evaluating the CoW.■