The purpose of this study is to compare the effects of a certain contrast agent (iodixanol) to two other commonly used contrast agents called Iopamidol (Isovue) and iohexol (Omnipaque), on heart and kidney safety in patients undergoing a cardiac CT angiogram. The investigators will evaluate whether iodixanol 320 (Visipaque®), an iso-osmolar agent, is better tolerated (flushing, injection site pain/warmth, headache, nausea) and provides equal image quality compared with iohexol 350 (Omnipaque®) and Isovue 370 during 64-slice multidetector computed tomography angiography (MDCTA). A fourth group (Visipaque 270) will be included, using low radiation dose technology during acquisition, to evaluate image quality, tolerability, and contrast enhancement compared to both Visipaque 320 and Omnipaque and Isovue. In addition to image quality, tolerability, and contrast enhancement, the investigators will evaluate heart rate, adverse events, reimaging, costs, and patient satisfaction among all four cohorts.