This study will compare the safety of coronary artery bypass grafting (CABG) with and without the use of heart-lung bypass during surgery (on- versus off-pump surgery). CABG carries a risk of stroke and of cognition problems (problems with thought processes) that may be caused by small strokes. This study will use magnetic resonance imaging (MRI) to determine whether the newer technique of off-pump CABG carries a lower risk of stroke than on-pump CABG. The study will also evaluate the relative risk (the risk of stroke in CABG patients exposed to the following factors compared to CABG patients who are not exposed to the following factors) of other factors for stroke and cognitive problems, such as atherosclerosis (hardening of the arteries), age, sex, pre-surgery intellectual function and performance, previous stroke, chronic kidney failure, diabetes, high blood pressure, carotid artery disease (narrowing of the neck arteries to the brain), peripheral blood vessel disease, and micro embolism (tiny blood clots that travel to the brain).