Circle of Willis. Three-dimensional computed tomography (CT) scan of blood vessels in the circle of Willis (centre) in the brain. This is a network of arteries at the base of the brain which supplies much of the brain with oxygenated blood. CT scanning uses X-rays to obtain slice images through the body, which can be combined to give a 3-D image. This allows study of the body's inter- nal structures without using invasive methods.