The Willis ring plays an important role in the collateral circulation especially in patients with internal carotid artery (ICA) stenosis. However conventional time of flight (TOF) MR angiography may only offer limited knowledge of the compensatory blood flow attributed to collateral circulation for patients with stenosis. Vessel selective arterial spin labeling allows the assessment of blood flow of a specific vessel. The purpose of this study was to quantify the influence of various anatomical types on the compensatory capacity of the Willis ring for patients with severe stenosis or occlusion using combined TOF MRA and vessel selective ASL method.
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