Atherosclerosis is a condition in which deposits of fat, cholesterol, and other substances build up along the inner walls of arteries; these deposits are known as plaque. As plaque builds up, it increases the risk of heart attack. Previous autopsy studies have shown that vulnerable plaques, a type of atherosclerotic plaque, are commonly found in people who have had a heart attack. Until recently it has been difficult to identify vulnerable plaques in people prior to their death. Many people who have coronary artery disease and are at risk of having a heart attack undergo angioplasty, a procedure that opens a narrowed or blocked blood vessel. During angioplasty, imaging devices are often used to take pictures of the inside of blood vessels. The most commonly used imaging device, an intravenous ultrasound (IVUS) catheter, is threaded through the blood vessels and uses sound waves to take pictures. An OFDI is a new type of catheter that takes more detailed pictures of blood vessel walls and plaques. The additional detail provided by OFDI images may improve detection of vulnerable plaques, which may help physicians identify people who are at high risk of having a heart attack. This study will use standard IVUS imaging and OFDI to examine vulnerable plaques within blood vessels of the heart and to evaluate any changes that occur over time in heart blood vessels and plaque.