Pulmonary embolism (PE) is a blockage of an artery in the lung. It usually occurs when a blood clot from a deep vein in the legs becomes dislodged and travels through the heart to the arteries of the lungs. Pulmonary embolism can be fatal if it is not treated quickly, and it can be difficult to diagnose. Doctors typically use history and physical examination, blood tests (d-dimer assays and arterial blood oxygen levels), ultrasonography or x-rays with dye injections of the leg veins (venograms), and lung radioisotope injections or computed tomography scans (CAT scans) of the arteries in the lungs to diagnose PE. They also may insert dye into the blood vessels of the lungs, by using a tube inserted in a leg vein and threaded through the heart, to see blockages in the arteries in the lungs (pulmonary angiography).