Besides the physical requirements listed previously, we can utilize a radionuclide only if a suitable molecule can be labeled with it. There are very important classes of mole­cules in the body that are so small that we cannot label them with any gamma-emitting radioisotope without altering their structure, i. e., they do not contain any atom with a suitable gamma-emitting radioisotope, so we could label them only by attaching a group of atoms that would significantly change their biological behavior.

Fortunately, there is another possibility for labeling many biologically important molecules: applying positron emitters (see Section 4.4.2). There are positron-emitting radioisotopes of the most important constituents of organic molecules: carbon, oxygen, nitrogen, and fluorine (see Table 12.6).