Evaluate Carotid Artery Plaque Composition by Magnetic Resonance Imaging in People Receiving Cholesterol Medication

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. People with atherosclerosis are at risk of developing coronary artery disease, in which plaque build-up occurs in the arteries that supply blood to the heart, and carotid artery disease, in which plaque build-up occurs in the arteries that deliver blood through the neck to the brain. These conditions can lead to blood clots, heart attack, and stroke. Research has shown that people who have more fat content in atherosclerotic plaque may have a higher risk of experiencing a heart attack or stroke. Treatments for atherosclerosis include lifestyle changes, medicines, and medical procedures or surgery. There are several medications that can aid people in controlling their cholesterol levels, including atorvastatin, a medication that inhibits the production of cholesterol; niacin, a B-complex vitamin that can reduce cholesterol levels in combination with dietary changes; and colesevelam, a medication that inhibits fat absorption. Using magnetic resonance imaging (MRI), this study will evaluate whether these medications, alone or in combination, can decrease the fat content of atherosclerotic plaques within the carotid arteries of people with coronary artery disease and carotid artery disease. This study will enroll people with coronary artery disease or carotid artery disease. Participants will be randomly assigned to one of the following 40-month treatment groups: * Group 1 participants will receive atorvastatin, placebo niacin, and placebo colesevelam each day. * Group 2 participants will receive atorvastatin, niacin, and placebo colesevelam each day. * Group 3 participants will receive atorvastatin, niacin, and colesevelam each day. At a baseline study visit, participants will undergo a blood collection and will receive dietary counseling that will focus on lowering cholesterol levels. They will also undergo an MRI scan of their carotid arteries. For the next 4 months, participants will attend monthly study visits for repeat blood collection and dietary counseling; for the subsequent 36 months, participants will attend study visits every other month. Repeat carotid artery MRI scans will occur at Months 12, 24, and 36. At three different times during the study, researchers will ask participants to record their food consumption for 3 consecutive days.