Nuclear medicine is based on radioactive materials and detectors that record their emitted radiation. Radioactive materials range from single elements, such as radioactive iodine, through more complex radiopharmaceuticals that are chemically designed to target a tissue or system, to radiolabelled macromolecules, especially proteins, and radiolabelled blood cells. Radiation detectors range from well-counters for the assay of radioactivity in small samples of biological fluids, through hand-held probes, whole body counters, gamma cameras, and PET/CT cameras. Image formation in a gamma camera is achieved by a collimator with multiple parallel holes, and in a PET camera by complete rings of radiation detectors that simultaneously record the two gamma photons that are emitted in exactly opposite directions to each other when a positron emitted from a PET source undergoes annihilation with an electron.