Atherosclerosis, a condition in which fatty deposits of plaque build up along the inner walls of arteries, is a condition that may increase the risk of having a heart attack. Previous studies have shown that the presence of a specific kind of plaque, known as vulnerable plaque, is often found in people who have had a heart attack. This study will use a new imaging technique called optical frequency domain imaging (OFDI) to examine the presence of vulnerable plaques in people with coronary artery disease.