Nuclear medicine is the use of open radionuclides for diagnostic and therapeutic purposes (radionuclide therapy). It also includes the use of other radioactive substances and nuclear physics techniques for functional and localization diagnostics. George de Hevesy (1885-1966) lived as a lodger and in 1923 suspected that his landlady was offering him pudding that he had not eaten the following week. He mixed a small amount of a radioactive isotope into the leftovers. When she served him the pudding a week later, he was able to detect radioactivity in a sample of the casserole. When he showed this to his landlady, she immediately gave him notice. The method he used made him the father of nuclear medicine. It became known as the tracer method, which is still used today in nuclear medicine diagnostics. A small amount of a radioactive substance, its distribution in the organism, and its path through the human body can be tracked externally. This provides information about various metabolic functions of the body. The continuous development of radionuclides has improved radiation protection. For example, the mercury compounds chloro-merodrin and chloro-merodrin were abandoned in the 1960s as substances were developed that allowed a higher photon yield with less radiation exposure. Beta emitters such as I and Y are used in radionuclide therapy. In nuclear medicine diagnostics, the beta+ emitters F, C, N, and O are used as radioactive markers for tracers in positron emission tomography (PET). Radiopharmaceuticals (isotope-labeled drugs) are being developed on an ongoing basis.