Variations in the origin of the vertebral artery and its level of entry into the transverse foramen diagnosed by CT angiography

The origin of the vertebral artery (VA) varies, though most VAs enter the transverse foramen (TF) of the sixth cervical vertebra. On computed tomography (CT) angiographic images, we evaluated the prevalence of variations of both VA origin and its level of entry into the TF. We retrospectively reviewed CT angiographic images of 2,287 patients obtained using either of two 64-slice multidetector CT scanners. All patients were Japanese and underwent scanning from the aortic arch to the intracranial region; most had or were suspected of having cerebrovascular diseases. The left VA (LVA) arose from the aorta between the left common carotid artery and left subclavian artery in 94 patients (4.1 %) and in other variations in 44 patients (1.9 %). The right VA (RVA) arose from the extreme proximal segment of the right subclavian artery in 72 patients (3.1 %) and in other variations in 14 patients (0.6 %). The LVA entered the sixth TF in 2,127 patients (93.0 %), and the RVA entered the sixth TF in 2,146 patients (93.8 %). Anomalous origin and anomalous entry level into the TF correlated strongly. The total prevalence of variation in the origin of the LVA was 6.0 % and of the RVA, 3.8 %. The total prevalence of variation in entry level into the TF was 7.0 % for the LVA and 6.2 % for the RVA. Recognition and reporting of these variations is important in interpreting CT angiography to prevent complications during surgery of the aortic arch or lower neck.