We have to face two contradicting requirements: we should administer short-lived radionuclides to patients (as discussed in Section 12.2.4), while radioisotopes are generally produced either in a nuclear reactor or by an accelerator, in most cases far from the location of the application. The best solution is the use of radioisotope generators, in which case its longer-lived radioactive parent element is transported. For example, to produce Tc-99m, its parent element, molybdenum-99, is used. A key point of a generator is a suitable solvent that selectively dissolves the daugh­ter element from a porous column, but not the parent element. Fortunately in the Mo-99!Tc-99m generators, physiological saline (NaCl) can be used as a solvent, so that the eluate is suitable for intravenous injection. (In other cases, we are not so lucky. For instance, before gamma cameras came into general use, indium-113m generators applied hydrochloric acid as an eluent that had to be neutralized before being administered to a human.)

12.2.6.1 Other Radionuclides

• Besides Tc-99m, Tl-201 as thallium chloride and Ga-67 in the form of gallium citrate are commonly used for gamma camera imaging.

• From among the radioisotopes of iodine, I-131 was first applied for imaging (see Table 12.3), but it has serious drawbacks: its gamma energy (with the highest peak at 364 keV) is too high, and its half-life (8 days) is longer than is generally desirable for imaging. Moreover, it is a beta emitter as well, increasing its radiation dose; that is why it is mainly used today for therapeutic purposes (and to measure iodine uptake before the therapy of hyperthyreosis). Iodide injected into the circulation primarily accumulates in the thyroid, which explains its application in measuring and imaging thyroid function already from the first half of the twentieth century (see more in Section 12.5.1). Sodium iodide labeled with I-131 was the first and is still the most common radioactive substance used for therapy, utilizing its beta radiation. Hyperthyreosis and thyroid cancer are the main indications.

• Another common radioisotope of iodine is I-123, as its 159 keV gamma energy and 13-h half-life make it a close runner-up after Tc-99m for use in gamma imaging. However, it is a cyclotron product, like Tl-201 and Ga-67, which makes it rather expensive.

• Radioisotopes of iodine, built into tyrosine, are suitable for labeling various protein molecules. For in vitro concentration measurements, I-125 is most commonly used. Its characteristic X-rays around 27 keV and gamma peak at 35 keV lead to a relatively low personnel dose, while its 60-day half-life allows a longer time for usage. In practice, it can only be used for about 6 weeks after labeling since radiolysis (chemical decomposi­tion caused by radiation) degrades the radiochemical purity of the preparation (the percentage of the radionuclide in the desired chemical form).