Factors Affecting Choice of Radiotracers:

Several factors as depicted in Figure should be considered while choosing a suitable radiotracer. Most important of these is the half-life of the radioisotope. It should not be too short or too long. Therefore, it could be in the range of a few minutes to a maximum of couple of years so in which there is enough activity present throughout the experiment for measurement of activity and good counting statistics. A common criterion could be that the half-life should be at least as long as the duration of the experiment.

Figure: Illustration of factors affecting the choice of radiotracer

Too long half-life means low specific activity and it may result in problems along with disposal, storage, long term environmental contamination and excessive radiation dosage to the worker. It might be noted that the half-life of a tracer is also decided through the usage. For example, if an iodine isotope is required for imaging thyroid gland then ¹²⁸I (t_½ = 25 min) might be used whereas for any chemical studies long lived isotopes such as ¹³¹I (t_½ = 8 d) or ¹²⁵I (t_½ = 59.7 d) should be recommended depending on their ease of availability. Same, using sodium isotope as biological marker in human studies, ²⁴Na (t_½ = 15 h) should be used whereas for chemical studies long lived ²²Na (t_½ = 2.6 y) may be used.