A crucial point of imaging methods is the amount of the contrast material or tracer necessary for obtaining a reasonable image quality (see Table 12.4). For example, if a contrast agent is used to enhance X-ray imaging, the amount needed may have physiological effects, and it may activate defense mechanisms (induce an immune reaction) or saturate a secretion channel. In contrast, we need such a small number of molecules of a radiotracer that it will not change or influence the studied function.

The radiotracers used for the in vivo diagnostic and therapeutic procedures of nuclear medicine are called radiopharmaceuticals (i. e., medicines that emit radi­ation). In fact, these are not medicines in the traditional sense since they do not have any effect on the patient as chemicals due to their extremely low concentra­tions. However, similar to normal pharmaceuticals, strict rules apply for tests to be carried out before they are allowed to be administered to humans. Moreover, their radiation may have biological effects—that is what we use for radioisotope therapy.

In general, we use molecules that either are present in the body anyway in their unlabeled form or behave similarly to those present. In this way, we can image and/or study the undisturbed function.