Qualitative phase contrast MRA in the normal and abnormal circle of Willis.

PURPOSE To determine the direction of blood flow in the circle of Willis using a 3-D phase contrast MR angiographic (MRA) technique with high spatial resolution. SUBJECTS Fifty healthy subjects and 15 patients with occlusive disease were studied using 3-D phase contrast MRA. RESULTS In the 50 normal subjects, 39 (78%) had detectable flow in one or both posterior communicating arteries. In 24 (48%) of these subjects, flow was detected in both posterior communicating arteries, whereas unilateral flow was detected in 15 (30%). In 36 (92%) of the 39 normal subjects, flow in the posterior communicating artery was from anterior to posterior with only 3 (8%) showing reverse flow from posterior to anterior. The A1 segment of both anterior cerebral arteries was identified in 100% of normal subjects with flow in the expected direction from carotid to the A2 segment. In patients with carotid occlusion, the pattern of flow in the circle of Willis was altered with reversed flow in the ipsilateral posterior communicating artery and sometimes in the ipsilateral A1 segment. An ipsilateral posterior communicating artery was present in 10 of 17 occluded carotid arteries, all showing reversed flow. CONCLUSION 3-D phase contrast MRA provides useful information about the hemodynamics of normal and abnormal blood flow in the circle of Willis.