Fig. 1.

The vertebral arteries are composed of four segments. V1, the extraosseous segment, starts at the origin of the vertebral artery and runs to the transverse foramen of the C6. V2 is the foraminal segment where the artery passes through the transverse foramina from C6 to C1. V3, the extraspinal segment begins at the foramen transversarium of C2. The vertebral artery then passes along the superior aspect of the posterior ring of C1, then twists antero-superior towards the foramen magnum where it pierces the dura. Then begins V4, the intradural segment which courses to the pontomedullary junction, where the basilar artery is formed