● OVERVIEW:
To the patient, all radiopharmaceuticals appear to be the same- a clear,colorless liquid. However, we know that different molecules are used to image different organs based on the organ function. For example, it is logical that labeled phosphates would be suitable for imaging bones, as uptake of inorganic phosphate and subsequent incorporation into bone tissue are well documented.

Additionally, insoluble radiopharmaceuticals such as Tc-MAA and Tc-SC are used to image lungs and liver/spleen, respectively, because it is well known that these two organs remove particles from the blood stream based entirely upon particle size. This uptake is totally unrelated to chemical composition. For example, Tc-99m oatmeal with a particle size distribution in the range of 0.1–2 mm would make an excellent liver/spleen agent.

In order for the mechanisms of localization to produce excellent quality images, other factors are important:

■ The radioisotope should be a pure gamma emitter, ideally 99mTc.
■ Gamma energy should be between 100-250 kev
■ The t_eff should ideally be about 1.5 times duration of test
■ It is very important to have a high target:non-target ratio

In order for the mechanisms of localization to produce excellent therapeutic results, other factors are important:

■ The radioisotope should be a pure bemitter
■ Energy should be high (> 1 MeV)
■ t_eff should be moderately long, e.g., days
■ Critical to have a high target:non-target ratio

● MECHANISMS OF LOCALIZATION
1. Capillary Blockade
2. Phagocytosis
3. Compartmental Localization
4. Active Transport
5. Chemisorption (also known as Physicochemical Adsorption)
6. Cell Sequestration
7. Exchange Diffusion
8. Simple Diffusion (also known as Passive Diffusion)
9. Antigen/Antibody Reaction
10. Receptor Binding
11. Metabolic Trapping