Nuclear medicine is a branch of medicine that uses the pharmaceuticals labeled with radionuclides (radiopharmaceuticals) in diagnosis and therapy. In diagnosis, radioactive substances are administered to patients and the radiation emitted is measured by gamma cameras. In therapy, radionuclides are administered to treat disease or provide palliative pain relief. Nuclear medicine differs from most other imaging modalities in that the tests primarily show the physiological function of the system being investigated as opposed to the anatomy. Radioimmunoassay (RIA) is a highly sensitive laboratory technique used to measure minute amounts of substances including antigens, hormones, and drugs present in the body. The substance or antigen (a foreign substance in the body that causes antibody production) to be measured is injected into an animal, causing it to produce antibodies. Serum containing the antibodies is withdrawn and treated with a radioactive antigen and later with a nonradioactive antigen. Measurements of the amount of radioactivity are then used to determine the amount of antigen present.