False-colour gamma camera scan (scintigram) showing the blood circulation in both normal human lungs (anterior view). The image is a map of the distribution of radioactivity, recorded by an external gamma camera, from a radioactive tracer which is injected intravenously prior to taking the scan. The tracer consists of microspheres tagged with the radioisotope technetium-99m. The radioactive microspheres are small enough to pass into the capillaries of the lung. The technique is used in the diagnosis of pulmonary embolism, where the pulmonary circulation becomes obstructed by a blood clot (an embolus).