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Examples and prevalence of circle of Willis anomalies that hamper collateral function. Prevalence is derived from studies by using an external diameter of 1 mm as a threshold for hypoplasia of collateral arteries. A1 indicates the precommunicating part of the anterior cerebral artery; M1, main trunk of the middle cerebral artery; P1, precommunicating part of the posterior cerebral artery; BA, basilar artery; and VA, vertebral artery.
