The veins are the blood vessels that return the blood to the heart. They do not have the pressure in them that arteries do, so their walls are thinner and there is not as much bleeding when they are punctured. The veins are not normally seen in an X-ray , so a contrast material is injected into the vein to make it visible. In this test, the contrast material is injected into the vein of the affected extremity. X-rays are a form of electromagnetic radiation (like radio waves or light). The wave length is very short and is capable of penetrating the body and producing a shadow picture of the structures inside the body. Some of the X-rays are absorbed by body tissues, while other X-rays will pass through. The denser tissues, such as bone, appear on the film as white. Less dense tissues, such as muscles, appear as gray; and air on the X-ray appears as black.