A catheter is fed into an artery in the groin or arm and advanced though the blood vessels to the heart. Dye is then injected so that the heart and blood vessels can be seen on X-ray.  X-rays and measurements are then taken, giving the doctors information about the heart’s condition and the exact nature of any narrowed blood vessels.  This allows them to plan the best form of treatment to prevent heart attacks and control any symptoms being experienced.