Pulmonary embolism. Coloured frontal computed tomography (CT) scan of an axial section through the chest of a 53-year-old patient with a pulmonary embolism (PE) causing an occlusion (blockage) of the right pulmonary artery. An embolism a blockage caused by a blood clot (embolus) that has dislodged and moved to a different area of the body, here to the pulmonary artery of the right lung. Small clots may cause shortness of breath, dizziness and chest pain. Large clots can be fatal, as the blood is quickly starved of oxygen.