Human lungs. Coloured gamma scan (scintigram) of healthy human lungs, the respiratory organs. The centres of the lungs (brown, pink & white) have absorbed more radioactive tracer than the outsides (blue & green). Gamma scanning involves introducing a radioactive isotope (radionuclide) into the body, in this case Technetium-99m, which accumulates in the target organ. The isotope emits gamma rays which are detected as flashes of light by a gamma camera. Isotopes are chosen for their ability to gather in cancerous or inflamed tissue to produce 'hot spots'. Gamma scanning produces slices" through tissues."