The main field of nuclear medicine today is imaging with gamma cameras. What can justify the in vivo use of radioactive preparations delivering radiation dose to humans? Other generally applied modalities of medical imaging, including ultrasound, X-ray, and X-ray computed tomography (CT), and most routine proce­dures of magnetic resonance imaging (MRI) are structural imaging methods. A pathological process is visible in these images only when it has already caused structural changes. For instance:

• The borders between tissues of different acoustic impedance (determined by the elasticity and density) can be seen in ultrasound images. An abnormal process will be visible only when it has already altered the structure of the tissue.

• X-ray (including CT) images will distinguish tissues with different radiation attenuation,

i. e., density. The easiest is to differentiate solid bones from soft tissues.

On the contrary, by applying radioactive tracers, we can follow the accumula­tion, secretion, metabolism, and excretion process of various molecules, so that a pathological process can be identified even in an early stage, when the structure of the investigated organ is not yet significantly different from its normal state. That is why the imaging methods of nuclear medicine are considered functional rather than anatomical.

Moreover, much smaller molar concentrations of radiotracers can be detected than the usual concentrations of contrast materials used for structural imaging

(see Table 12.4); thus, the application of a radiotracer does not interfere with or change the function of the organ investigated.

PET studies contribute about 6% to nuclear medical imaging, and the distribu­tion of gamma camera studies depends on both health-care protocols and the reimbursement policy of a country (see Table 12.5 and Figure 12.3).