The demands of in vitro isotope diagnostic procedures are different from those of imaging as follows:

• A longer half-life is desirable, so labeled products can be stored for a longer time.

• A wider range of radiation energy is acceptable. A lower gamma energy is even prefera­ble so that the staff dose can be decreased. The reason is that we can ensure the same measurement geometry of the radioactive specimen (see Chapter 14), i. e., both the standards and the samples are in the same type of (plastic) vial, volume, and medium. Therefore, the attenuated fraction of the radiation is the same, and the accuracy of the measurement is not reduced. Iodine-125 is the radionuclide most frequently used for in vitro diagnostic procedures.

• For concentration measurements, beta emitters can also be used, but for this procedure, a special measuring technique, the so-called liquid scintillation counting, is required, as beta radiation is absorbed in the sample and its vial, so we cannot use an external radia­tion detector to measure it (see also Sections 5.3.4, 5.3.5, and 14.2.1).