When someone has a heart attack, part of the heart muscle loses its blood supply and becomes damaged. Doctors have long wanted better ways to understand exactly how scar tissue changes the heart’s electrical system. Some could show how electrical signals moved through the heart, while others could identify damaged tissue. The system could also identify healthy heart muscle and the border area where healthy and damaged tissues meet. The combined images revealed that dangerous electrical signals often began near the edge of the scar rather than inside it.