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Extracranial-intracranial (EC-IC) bypass surgery is performed for patients with 2 types of cerebrovascular disease: 1) internal carotid artery (ICA) occlusion and 2) cerebral aneurysm requiring ligation of the ICA. Flow patterns of the cerebral arteries may vary after EC-IC bypass, because the flow from the bypass and the collateral flow through the circle of Willis compensate for the decrease in flow from the occluded ICA. Conventional X-ray angiography has been the standard method of assessing hemodynamics after EC-IC bypass surgery but is invasive and requires irradiation and an iodinated contrast agent. Thus, X-ray angiography is not suitable for the long-term follow-up of patients after EC-IC bypass surgery. We use time-resolved 3-dimensional phase-contrast (4-dimensional [4D] Flow) magnetic resonance imaging (MRI) for flow analysis before and after EC-IC bypass surgery. Unlike X-ray angiography, 4D Flow MRI is noninvasive and does not require injection of a contrast agent. We have used 4D Flow MRI to assess intracranial hemodynamics in 19 patients who underwent EC-IC bypass surgery. With 4D Flow MRI, the patency of EC-bypass arteries was identified in all patients. Retrograde flow at the M1 segment of the middle cerebral artery (MCA) from the EC bypass was observed in 10 patients, whereas anterograde flow was identified in 9. We were able to comprehensively assess the flow crossing from the circle of Willis to the bypass, and visualized the complicated flow at the bifurcation of the MCA. Our results indicate that 4D Flow MRI is feasible for following up patients after EC-IC bypass surgery.

Four-dimensional Flow Magentic Resonance Imaging Assessment of Hemodynamics in Patients after Extracranial-Intracranial Bypass Surgery