An angiographic study of the distal pa of the circle of Willis requires access to the veebral aery. Access to this aery can be obtained the:

The suboccipital triangle lies in the neck at the base of the skull. The suboccipital triangle is bounded by the inferior oblique, rectus major, and superior oblique muscles. The veebral aery can be found within the suboccipital triangle, lying on the posterior arch of the atlas, lateral to the midline, typically at about the level of the lower poion of the ear lobe. Care should be taken in accessing the vessel, since the first cervical nerve lies between the veebral aery and the posterior arch of the atlas.The anterior triangle of the neck is in the anterior neck, and is bounded by the neck midline, the mandible, and the sternocleidomastoid muscle. The anterior triangle of the neck is subdivided into the muscular, carotid, submandibular, and submental triangles.The small muscular triangle lies between the superior belly of the omohyoid and the sternohyoid muscle. The posterior triangle of the neck is on the side of the neck, and is bounded by the sternocleidomastoid muscle, trapezius, and the middle third of the clavicle. Ref: Waxman S.G. (2010). Chapter 12. Vascular Supply of the Brain. In S.G. Waxman (Ed), Clinical Neuroanatomy, 26e.