False-colour angiogram of the left carotid artery, revealing a stenosis (obstruction) at the point of the bifurcation of the common carotid into the internal & external carotid arteries. The obstruction is represented by a lack of contrast (appearing as a pale white notch) immediately to the left of the body of the uppermost cervical (neck) vertebrae visible. The straight red line running at an angle into the common carotid (bottom left) is the catheter used to introduce the radio-opaque contrast medium which has outlined these arteries.