Deep vein thrombosis. Coloured angiograms (X-rays) taken from the front (left) and side (right) of a leg showing deep vein thrombosis in the calf. One thrombus (abnormal clot) is blocking the flow of blood (purple/pink) in the vein immediately to the right of the tibia (shin bone, green) in the view at left. Deep vein thrombosis is most common in the legs, occurring when a thrombus forms in undamaged deep-lying veins. The restricted blood flow may cause pain, swelling and tissue death. The angiograms were produced by injecting a radio- opaque dye into the blood system.