Chapter 37 Radionuclide imaging

37.1 Aim

The aim of this chapter is to introduce the reader to the basis of the technology used in radionuclide imaging. The information available on a nuclear medicine scan will be compared with the information available on other forms of medical imaging.

37.2 Basic concept of radionuclide imaging

The basic concept of radionuclide imaging involves a three-stage process:

1. A suitable radionuclide is introduced into the patient – this is usually an artificially produced radionuclide.

2. The radionuclide is allowed to concentrate in a specific organ.

3. The organ is scanned using an appropriate scanner, e.g. a gamma camera.

37.3 Production of artificially produced radionuclides

There are three common ways of producing artificially produced radionuclides which are then used in radionuclide imaging. These are:

1. irradiating materials in a nuclear reactor

2. using a technetium generator

3. using a medical cyclotron.

37.3.1 The nuclear reactor

The nuclear reactor (or pile) produces heat energy by controlled fission (see Sect. 19.9). The heat generated within the reactor raises the temperature of a coolant which in turn is used to heat water to produce steam. The steam can then drive very powerful electric generators. This is the basis of nuclear power stations.

A simplified diagram of such a reactor is shown in Figure 37.1 (see page 272). The controlled fission is produced by using the neutrons of fissile decay to produce further fission in other atoms. Thus, a sustained reaction can be set up where one neutron released during the fission of a nucleus will interact with another nucleus to produce fission of that nucleus with the release of further neutrons.

Figure 37.1 Simplified diagram of a pressurised water nuclear reactor.

Nuclear reactors are important in radionuclide imaging in that they allow us to insert samples into the neutron flux within the reactor. This results in neutrons being inserted into nuclei to allow the manufacture of artificial radionuclides. An example of this is that stable molybdenum-98 can be made to absorb a neutron to produce the radionuclide molybdenum-99. The reaction may be shown using the equation below:

Equation 37.1

37.3.2 The technetium generator

It is not feasible to produce nuclei with very short half-lives at a remote site and then transport these to the hospital. Such radionuclides are produced at the hospital’s radiopharmacy either by the use of a technetium generator or by the use of a medical cyclotron.

The technetium generator used in nuclear medicine is an important example of the production of artificial radionuclides. As mentioned in Section 37.3.1, if molybdenum-98 is placed in a neutron stream, the nuclei of the molybdenum atoms can be made to absorb the neutrons to produce molybdenum-99. The capture of a neutron raises the energy of the resulting molybdenum-99 nuclei and each loses this energy by the prompt emission of a gamma-ray.

A molybdenum-99/alumina column is in the centre of the generator, as shown in Figure 37.2 (see page 272). The molybdenum-99 has a half-life of 67 hours and decays to form technetium-99m by β⁻-particle emission, as shown below:

Figure 37.2 Principal features of a technetium-99m generator.

Equation 37.2

The is eluted (or flushed) from the generator at regular intervals as sodium pertechnetate. This radionuclide, which is in liquid form, may be used for a number of radionuclide imaging situations. The decays to by the emission of a gamma-ray of energy 140 keV. The metastable radionuclide has a half-life of 6 hours. Clearly, after a period of time, the activity of the molybdenum-99, and hence its ability to produce technetium 99m, will be reduced and the technetium generator must have its molybdenum-99/alumina column replaced.

A number of other radionuclides used in nuclear medicine can be produced from stable materials when they are bombarded with particles but further discussion about their production is beyond the scope of this section

37.3.3 Production of radionuclides using a cyclotron

The type of cyclotron used in nuclear medicine to produce artificial radionuclides by the bombardment of stable substances will briefly be described. A simple diagram of such a device is shown (see Fig. 37.3) (see page 272). The cyclotron consists of an evacuated cylinder which has an ion source placed at its centre. Ions from this source are influenced by strong axial and radial magnetic fields. This causes acceleration of the ions in circular paths of increasing radius. This ion beam achieves significant velocity and can be made to interact with materials placed at the exit port of the cyclotron. This interaction causes nuclear changes in these materials and we can produce neutron-deficient nuclei (see Sect. 19.5.1) which are capable of positron emission. Such materials form the basis of the radiopharmaceuticals used in positron emission tomography (PET) scanning. Figure 37.4 (see page 272) shows a photograph of such a medical cyclotron.