Blood clots (thrombi) sometimes form in leg veins (deep-vein thrombosis, or DVT) and can then break free, eventually coming to rest in the blood vessels of the lungs. This problem, called pulmonary embolism, can be fatal. It is important to diagnose DVT rapidly and accurately because treatment with blood thinners is very effective in preventing complications. Some tests for DVT are painful, and they may provide uncertain answers that often require additional tests to be sure of the diagnosis. To measure the reliability of the answers provided by a new test, researchers compare its results to a diagnostic “gold standard,” which in the case of DVT is contrast venography. In performing contrast venography, doctors inject the leg veins with a liquid that shows the veins on an x-ray and outlines any clots. Magnetic resonance imaging is a rapid and accurate technique for visualizing structures inside the body. It has recently been used to detect thrombi without the need to inject anything into the body; this technique is called magnetic resonance direct thrombus imaging (MRDTI).