Circle of Willis collateral flow investigated by magnetic resonance angiography.

BACKGROUND AND PURPOSE The circle of Willis (CW) is considered an important collateral pathway in maintaining adequate cerebral blood flow in patients with internal carotid artery (ICA) obstruction. We aimed to investigate the anatomic variation of the CW in patients with severe symptomatic carotid obstructive disease and to analyze diameter changes of its components in relation to varying grades of ICA obstruction and in relation to the presence or absence of (retrograde) collateral flow. METHODS Seventy-five patients with minor disabling neurological deficits and with ICA stenoses or occlusions were categorized into 4 groups according to the severity of ICA obstruction. This patient population reflected a relatively favorable subgroup of cerebral infarction (considering their minor neurological deficits). All subjects underwent magnetic resonance angiography, including magnetic resonance angiography sensitive to flow direction. CW morphology and the size of its components were determined and compared with those values in control subjects (n=100). RESULTS Compared with control subjects, patients demonstrated a significantly higher percentage of entirely complete CW configurations (55% versus 36%, P=0.02), complete anterior configurations (88% versus 68%, P=0.002), and complete posterior CW configurations (63% versus 47%, P=0.04). Patients with severe ICA stenosis did not show significantly increased CW vessel diameters. Patients with ICA occlusion demonstrated a high prevalence of collateral flow through the anterior CW and significantly increased diameters of the communicating channels. Patients with bilateral ICA occlusion relied on collateral flow via the posterior CW and demonstrated a bilateral increase in posterior communicating artery diameters (P<0.05). CONCLUSIONS The anatomic and functional configuration of the CW reflects the degree of ICA obstruction.