Decreased brain blood flow. 3D computed tomography (CT) scan of the rear of the head of a 48-year-old patient, showing the base of the skull (top) and the cervical spine of the neck (bottom). Here, the vertebral arteries appear smaller than usual at the site of the foramen magnum (highlighted, centre), suggestive of vertebrobasilar insufficiency. VBI (or vertebral basilar ischaemia) is a temporary set of symptoms caused by decreased blood flow in the posterior circulation of the brain.