For clinical educational purposes only. An automated decision tree for evaluating risk of posterior circulation stroke in patients with vertebrobasilar disease. A physician will use the app as a reference, to evaluate an individual patient's blood flow in the basilar and posterior cerebral arteries according to a published algorithm. The algorithm is used to determine a patient's flow status based on the amount of blood flow distal to a stenosis or occlusion within the vertebrobasilar circulation. Flow status is associated with a patient's future risk of stroke.