Thrombosis. Coloured venogram (X-ray) showing deep vein thrombosis in the leg's popliteal vein (red, down right). The tibia (shin-bone, olive green) is beside the fibula (centre left). A large thrombus, or abnormal blood clot (dark red, centre right), is in the calf. There are also other smaller clots (dark red) causing irregular blood flow in branching veins of the leg. In thrombosis, a blood clot forms in an undamaged blood vessel, causing pain and tissue damage. Although thrombi often break up on their own, drugs can aid this process and surgery may be needed. A venogram is produced by injecting an X-ray opaque dye into a vein.