Additional Study Information: The purpose of this study is to evaluate two different types of devices that measure pressure across the coronary arteries, which are vessels that supply blood to the heart. This is being done in order to see if they are equally accurate or if one performs better than the other. These devices are called Fractional Flow Reserve (FFR) . The devices that will be used are the St. Jude Medical Aeris FFR Pressure wire system and the ACIST Navvus pressure catheter system. Both devices are approved for commercial use by the U.S. Food and Drug Administration (FDA). These devices are routinely used during a procedure called an angiogram where there is an X-ray test that uses a special dye and camera to view how blood flows through an artery. This study is for patients who are having an angiogram and their physician choose to measure their blood flow using fractional flow reserve (FFR).