Angiography is used to obtain diagnostic information about the blood vessels that carry blood to various parts of the body, particularly whether the vessel is blocked by a blood clot or narrowed by atherosclerosis.
For this procedure, the doctor inserts a slender catheter into an artery in the groin and threads it through to the area being evaluated. When the brain is being evaluated (called cerebral angiography), the catheter is threaded to the aorta, then to the large arteries that carry blood to the brain (carotid and vertebral arteries).
After contrast agent is injected through the catheter, a technician takes x-rays of the arteries and veins being evaluated (in the head for cerebral angiography), which are outlined by the contrast agent.
(Video from Center for Biomedical Communications, College of Physicians and Surgeons, Columbia University.)