The average diameter of the normal right vertebral artery is 3.81 ± 0.47 mm, and the normal diameter of the left vertebral artery is 3.88 ± 0.47 mm. In general, a measurement of a full diameter of <2.0 mm on one side is called a physiological full stenosis. ＞If the diameter of the left vertebral artery is 3.88 ± 0.47 mm, the normal diameter of the left vertebral artery is 3.88 ± 0.47 mm. Generally, if there is occlusion or stenosis of the vertebral artery on the opposite side, this segment of the vertebral artery will widen compensatingly, but the widening blood flow rate will also increase compensatingly, and the vertebral artery will merge upward into the basilar artery. If there is a lesion in the intracranial carotid artery, the posterior circulation supplies the anterior circulation, and the blood flow compensates, the entire diameter of the canal will also widen.