Читать книгу Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology - Rachel A. Powsner - Страница 62

The formal mathematical description of time‐activity behavior for parent and daughter radionuclides is complicated because it involves the competition between the accumulation (caused by decay of the parent) and decay of the daughter. The plot of the curve describing the amount of daughter nuclide in a generator following elution has two segments. The first segment traces the period of rapid accumulation of the daughter nuclide and lasts for approximately four half‐lives of the daughter nuclide (which for ^99mTc is approximately 24 hours). The second segment of the curve traces what is called the period of equilibrium, during which time the amount of daughter nuclide decreases as the parent nuclide decays.

Medical radionuclide generator systems, for practical reasons, have parent half‐lives longer than their daughters—in most cases much longer. We classify generators into two groups: those in which the parent half‐life is 10 to 100 times that of the daughter and those in which the parent half‐life is more than 100 times that of the daughter. In the first group, the activity of the daughter during equilibrium decreases perceptibly over time (when time is measured in units of daughter half‐lives). This is called transient equilibrium. On the other hand, the equilibrium segment of the curve for the second group is relatively flat. This is called secular equilibrium.

Figure 3.1 ⁹⁹mTechnetium generator.

Table 3.1 Characteristics of three commonly used generators

Generator (Parent–Daughter)	Clinical uses of daughter nuclide	Half‐life of parent (T1/2p)	Half‐life of daughter (T1/2d)	T1/2p/T1/2d
99Mo–99mTc (molybdenum‐99–technetium‐99m)	Used in most radiopharmaceuticals for nuclear studies	66 h	6 h	11
82Sr–82Rb (strontium‐82–rubidium‐82)	Cardiac perfusion imaging (PET)	25.5 days	75 s	29,000
68Ge–68Ga (germanium‐82–gallium‐82)	Neuroendocrine imaging (PET)	271 days	68 min	5,800