MRA scan from a patient with a large tangle of abnormal blood vessels inside of the brain (*) called an arteriovenous malformation (AVM).
The arterial supply to the brain arises from paired carotid and vertebral arteries in the neck. These four vessels continue into the head and divide into separate anterior, middle and posterior cerebral arteries that provide oxygen and nutrients to different regions of the brain. These vessels are interconnected at the base of the brain through a network of arteries called the circle of Willis. The principal veins within the head, the dural venous sinuses, collect blood that has passed through the brain. Blood vessels may cause symptoms by becoming enlarged or narrowed, occluded, or by supplying vascular malformations in or around the brain.
Image source: Google Knol (Distributed under the terms of the Creative Commons Attribution License.)