Human kidneys. Coloured gamma scan (scintigram) of healthy human kidneys, which excrete urine and regulate the blood and electrolyte balance. The kidneys' centres (orange/white) have absorbed more radioactive tracer than their outsides (yellow/ brown). 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, producing 'hot spots'. Gamma scans are tissue slices"."