The range (path length) of various types of radiation in body tissues primarily determines the areas of possible application. Body tissues are practically water equivalent at the gamma energies used for imaging (see Tables 12.1 and 12.2).

12.2.4.1 Selection of Radionuclides for Imaging

• Only electromagnetic radiation (gamma — or X-rays) can be detected from outside the patient’s body, as beta radiation (and alpha even more) is adsorbed in a few millimeters of body tissue at most (Table 12.1).

• Besides, gamma energy should be in the range of 60—500 keV. The majority of lower — energy photons will be attenuated inside the patient’s body, while higher-energy photons most likely fly through the detector without any interaction; the counting efficiency is low in both cases.

• An important aspect is the half-life of the radionuclide: several hours (or a few days in some cases) are preferred, so that radioactive material will disappear from the patient’s body shortly after the imaging is completed, thus limiting the radiation dose. If a shorter — lived radionuclide was used, a large proportion of the radioisotope would decay during the procedure of labeling the selected molecule, thus increasing the cost of production.

• If the radionuclide also emits alpha or beta radiation, they unnecessarily increase the patient’s radiation dose while not contributing to image formation.