False-colour venograph (phlebograph) of the human head, showing the network of normal veins that drains blood from the brain; the veins are coloured red. Venography is the X-ray examination of a vein that has been outlined by the prior injection of a radio-opaque contrast medium. The technique is used to determine whether damage, obstruction or abnormal communication with other vessels is occurring; abnormalities are indicated when the vein is incompletely filled by the contrast medium, or when the medium appears to be leaking from the vein.