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The vertebral arteries (VAs) are originated from the subclavian arteries and are major arteries for posterior circulation. The left and right VAs are typically described as having 4 segments each (V1 through V4), the first 3 of which are extracranial [1]: the V1 segments extend cephalad and posteriorly from the origin of the vertebral arteries between the longus colli and scalenus anterior muscles to the level of the transverse foramina, typically adjacent to the sixth cervical vertebra. The V2 segments extend cephalad from the point at which the arteries enter the most inferior transverse portion of the foramina to their exits from the transverse foramina at the level of the second cervical vertebra. These segments of the left and right VAs therefore have an alternating intraosseous and interosseous course, a unique anatomic environment that exposes the V2 segments to the possibility of extrinsic compression from spondylotic exostosis of the spine. Small branches from the V2 segments supply the vertebrae and adjacent musculature and, most importantly, may anastomose with the spinal arteries. The V3 segments extend laterally from the points at which the arteries exit the C2 transverse foramina, cephalad and posterior to the superior articular process of C2, cephalad and medially across the posterior arch of C1, and then continue into the foramen magnum. Branches of the V3 segments typically anastomose with branches of the occipital artery at the levels of the first and second cervical vertebrae. The V4 segments of each vertebral artery extend from the point at which the arteries enter the dura to the termination of these arteries at the vertebrobasilar junction. Important branches of the V4 segments include the anterior and posterior spinal arteries, the posterior meningeal artery, small medullary branches, and the posterior inferior cerebellar artery (PICA) [1].

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