Generally speaking, radionuclides used for imaging in nuclear medicine are not harmful and decay completely into harmless substances within a few hours after intravenous injection. The dose of radionuclides for medical use is strictly controlled by international and national standards, and the dose is very low. For the human body, it is equivalent to the radiation dose of two chest films or one enhanced CT examination. The biological effects caused by ionizing radiation of radionuclides on human body are mainly divided into deterministic effects and random effects. Deterministic effect means that the magnitude of biological effect is proportional to the dose and has a threshold value; random effect means that the biological effect is not clearly correlated with the dose and has no threshold value, which also refers to the teratogenic or malignant tumor effect caused by DNA damage caused by radiation. There are two sides to everything, doing small doses of radionuclides sometimes have health effects on the body instead, often called small doses of health care.