Pulmonary emboli are small pieces of blood clots. They usually break off from larger clots in the veins of the legs and travel through the circulatory system to the lungs. The emboli can block blood flow to the lungs and prevent oxygen from reaching the tissues. They can cause shortness of breath, chest pain, coughing up of blood, and death. Pulmonary emboli can be diagnosed in many ways, including by ultrasonography or dye tests to look for the source of clots in the legs, special blood tests (d-dimer assays), and scan or dye tests of the lung. Helical computed tomography (CT) is one way to scan the lungs for pulmonary emboli. This test uses a computer to display multidimensional images made from x-rays that pass through the body as a CT machine is rotated around the patient. Although helical CT is widely used, some studies suggest that helical CT alone is not a good way to rule out pulmonary emboli.