As an aid to the diagnosis of disease (diagnostic radiopharmaceuticals)

 In the treatment of disease (therapeutic radiopharmaceuticals).

 Radiopharmaceuticals used in tracer techniques for measuring physiological parameters (e.g. ⁵¹Cr-EDTA for measuring glomerular filtration rate)

 Radiopharmaceuticals for diagnostic imaging (e.g. ^99mTc-methylene diphosphonate (MDP) used in bone scanning).

Alpha-emitters

Alpha-decay is the process whereby a nucleus emits a helium nucleus, or alpha-particle. This commonly occurs with heavy nuclei (e.g. ²²⁶Ra:²²⁶₈₈Ra→²²²₈₆Rn+alpha).

Because they are heavy and positively charged, alpha-particles travel only short distances in air (~5 mm) and only micrometre distances in tissues. Their ionizing nature would result in a highly localized radiation dose if taken internally and hence they tend not to be used as diagnostic radiopharmaceuticals but may have a place as therapeutic agents.

Some alpha-emitters (e.g. ¹³⁷Cs) when encapsulated are used as sealed sources, emitting X-rays or gamma-rays for radiotherapy applications. Here the body is exposed to radiation externally in an attempt to treat malignant tumours.

Beta-emitters

Beta-decay occurs in two ways, one that involves the emission of a negatively charged beta⁻-particle, or electron, and the other that involves the emission of a positively charged beta⁺-particle, or positron.

Electron capture

Nuclei that are proton rich may, as an alternative to positron emission, capture electrons from the atom’s electron orbital. This process results in the transformation of a proton to a neutron within the nucleus. The subsequent rearrangement of the electrons orbiting the nucleus results in a characteristic emission of X-rays or gamma-rays (e.g. ¹²³I:¹²³₅₃I +  electron→¹²³₅₃Te+gamma).

Radionuclides which decay by electron capture are useful in diagnostic imaging since they emit gamma-rays; examples are given in Table 45.1.

Isomeric transition

Some radionuclides exist for measurable periods in excited, or isomeric, states prior to reaching ground state. This form of decay involves the emission of a gamma-ray and is known as isomeric transition. When radionuclides exist in this transitional state, they are known as metastable, which is denoted by the letter ‘m’ and written thus: ^99mTc.

A simplified decay scheme for ^99mTc-technetium is shown in Figure 45.1 where ^99mTc’s parent radionuclide, molybdenum (⁹⁹Mo), decays by beta⁻-emission to the ground state ⁹⁹Tc either directly or indirectly.

The indirect route, which is the most common, involves the isomer ^99mTc, which in turn decays from its metastable state to ⁹⁹Tc by isomeric transition.

Radionuclides which decay by this process are used in diagnostic imaging since they emit gamma-rays (see Table 45.1). It should be noted that ^99mTc is the most widely used radionuclide in hospital radiopharmacy today, making up the radionuclide component of around 90% of the radiopharmaceuticals produced. For these reasons the production processes for ^99mTc-radiopharmaceuticals will be especially emphasized (see below).

 It has a 6-h half-life (T_1/2); long enough to allow imaging to take place in the working day, while also being short enough that patients are not radioactive for long periods (in 24 h, or 4 half-lives, the radioactivity will have decayed by 94%)

 ^99mTc emits gamma-rays of 140 keV energy: ideal for use with the modern gamma-camera

 There are no particulate emissions that, if present, would add to the patient’s radiation dose

 By purchasing a device known as a ⁹⁹Mo/^99mTc -generator, ^99mTc can be made readily available to the hospital site in a sterile and pyrogen-free form

 ^99mTc has versatile coordination chemistry and will allow a large number of ligands to complex with it. By using different ligands in the radiopharmaceutical’s formulation, a wide range of radiopharmaceuticals can be prepared in the radiopharmacy, providing for the many different investigations carried out in nuclear medicine departments (Table 45.2).