Ultrasound is used to screen for blood clots in blood vessels. Because the blood vessels are relatively superficial, ultrasound examination can reveal the size and extent of the thrombus and the surrounding blood flow, and ultrasound spectroscopy can reveal whether there is blood flow signal inside or around the thrombus. The examination of thrombus also requires CT, for example, a contrast agent is injected into the blood vessel and then a CT scan is performed, which can show filling defects in both arterial and venous vessels, which should indicate that there is a thrombus inside the vessel.