Duplex ultrasonography is a painless, 30-minute procedure used to “map” the veins in your leg in preparation for a procedure such as bypass graft surgery or invasive testing. As in other sonography procedures, a special gel is applied to the skin above the area to be examined and a hand-held transducer device is glided over that location as it emits sound wave energy to create a picture on a computer monitor. The person performing the test measures the vein or veins in your leg and marks their location on your skin for the vascular surgeon’s review and future guidance. Duplex ultrasonography is a special kind of ultrasonography for evaluating the circulatory system, including the peripheral arteries and veins. In addition to helping visualize the structure of blood vessels, it allows your physician to see the flow of blood through an artery so as to determine the degree to which the vessel is blocked or narrowed.