Knee thrombosis. Coloured X-ray angiogram of a thrombus (blood clot, brown, upper right) in the popliteal artery (pink) where it passes a knee. The leg bones and kneecap (left) are green. A large and severe thrombus (as here) may cut off the supply of blood to parts of the body, or parts may detach and travel to the heart or lungs where the fragments may cause death. Peripheral blood vessels, either side of the thrombus, maintain the blood supply to the lower part of the leg. Clots can arise in diseased blood vessels, and can be treated by the use of clot-dissolving drugs. X-ray angiography injects a radio-opaque contrast medium to highlight blood vessels on an X-ray.