Pulmonary emboli are pieces of blood clots that break off from clots in the legs (and sometimes other parts of the body) and travel through the circulatory system to the lungs. Pulmonary emboli can block blood flow to the lungs, which in turn can prevent sufficient oxygen from getting to the tissues. They can cause shortness of breath, chest pain, coughing up blood, and death. Pulmonary emboli can be diagnosed in many ways, including ultrasound tests to look for the source of clots in the legs, as well as special blood tests, lung scans, and dye tests of the arteries in the lung (angiography). Helical computed tomography (CT) is a relatively new way to examine the lungs for pulmonary emboli. This test uses a computer to display two- or three-dimensional images made from x-rays that pass through the body as a CT machine is rotated around the patient. Helical CT scans are widely available. However, their accuracy in detecting pulmonary emboli is not established.