Renal artery stenosis is a narrowing of the artery that supplies blood to the kidney. It can cause kidney dysfunction and high blood pressure. One of the most accurate ways to detect renal artery stenosis is to x-ray the renal arteries after injecting them directly with dye. This test, called intra-arterial digital subtraction angiography, or DSA, is invasive and sometimes dangerous. Thus, physicians often use alternative tests, such as computed tomographic angiography (CTA) and magnetic resonance angiography (MRA), to detect renal artery stenosis. These tests show pictures of arteries that have been injected with very small amounts of dye. Computed tomographic angiography uses x-ray beams and MRA uses radio waves and a magnetic field to take the pictures. Unfortunately, few studies address how well either of these tests detects renal artery stenosis.