Like an angiogram, a contrast dye has to be injected, in this case continuously, into the area to be studied. The dye is injected into a vein or into the bone marrow. The flow of dye has to be constant. In the vast majority of cases, this is accomplished by inserting a catheter into a vein in the groin and working it up to the area to be studied. As such, it is an invasive procedure.
It is the definitive procedure to confirm deep-vein thrombosis, but even in this case it is rarely used. It is more often used to distinguish clots from other obstructions, to evaluate congenital problems, to see how the valves in deep leg veins are working (often definitive to show varicose veins), or to identify a vein that would be suitable to graft onto an artery.