Groch M W
Department of Nuclear Medicine, Northwestern University Medical School and Northwestern Memorial Hospital, Chicago, IL 60611, USA.
Radiographics. 1998 Sep-Oct;18(5):1247-56; quiz 1245-6. doi: 10.1148/radiographics.18.5.9747617.
When a parent radionuclide decays to its daughter radionuclide by means of alpha, beta, or isomeric transition, the decay follows an exponential form, which is characterized by the decay constant lambda. The decay constant represents the probability per unit time that a single radioatom will decay. The decay equation can be used to provide a useful expression for radionuclide decay, the half-life, the time when 50% of the radioatoms present will have decayed. Radiotracer half-life has direct implications in nuclear imaging, radiation therapy, and radiation safety because radionuclide half-life affects the ability to evaluate tracer kinetics and create appropriate nuclear images and also affects organ, tumor, and whole-body radiation dose. The number of radioatoms present in a sample is equal to the activity, defined as the number of transitions per unit time, divided by the decay constant; the mass of radioatoms present in a sample can be calculated to determine the specific activity (activity per unit mass). The dynamic relationship between the number of parent and daughter atoms present over time may lead to radioactive equilibrium, which takes two forms--secular and transient--and has direct relevance to generator-produced radionuclides.
当母体放射性核素通过α衰变、β衰变或同质异能跃迁衰变为其 daughter 放射性核素时，衰变遵循指数形式，其特征在于衰变常数λ。衰变常数表示单个放射性原子在单位时间内衰变的概率。衰变方程可用于给出放射性核素衰变、半衰期（即现存放射性原子中有50%发生衰变时的时间）的有用表达式。放射性示踪剂的半衰期在核成像、放射治疗和辐射安全方面具有直接影响，因为放射性核素的半衰期会影响评估示踪剂动力学以及生成合适核图像的能力，还会影响器官、肿瘤和全身的辐射剂量。样品中存在的放射性原子数等于活度，活度定义为单位时间内的跃迁数除以衰变常数；可以计算样品中存在的放射性原子质量以确定比活度（单位质量的活度）。随着时间推移，母体和 daughter 原子数之间的动态关系可能导致放射性平衡，放射性平衡有两种形式——长期平衡和暂时平衡——并且与发生器产生的放射性核素直接相关。
