Non-contrast MRA techniques are felt to provide an avenue for safer, effective vascular imaging in patients both with and without kidney dysfunction. In this trial we propose to validate the contrast-free MRA technique, T-SLIP (time-spatial labeling inversion pulse), for diagnosing RAS. This study will compare T-SLIP to the reference standard of contrast-enhanced CTA and is designed to test its efficacy, reproducibility, reliability, and safety.