Nuclear medicine

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Chapter 16 Nuclear medicine

Vicki Major, Marc Griffiths

The radiation dose from any diagnostic procedure should be as low as reasonably practicable, consistent with diagnostic results.¹

This is governed in nuclear medicine by the administration of strictly controlled amounts of radioactivity under a licensing arrangement and the use of optimum imaging conditions.²

Any person working within the nuclear medicine environment should have an awareness of the practical radiation protection considerations.

Nuclear medicine studies require the administration of less than a microgram of the radioactive substance under test. This has the advantage that it does not interfere with the physiological pathways but demonstrates their function.

The radioactive substances may be given in one of two forms: in their radionuclide form or attached to a drug and known as a radiopharmaceutical.

The radiopharmaceutical or radionuclide determines which system or organ the radioactive substance targets.

Most radioactive substances used in diagnostic imaging emit gamma radiation, which can subsequently be detected externally to the patient, and an image is produced of the uptake in the specific area.

Nuclear medicine studies can often detect certain disorders earlier than other diagnostic imaging procedures because they rely on functional rather than structural changes.

INTRODUCTION

Nuclear medicine is concerned with providing diagnostic information about patients following the administration of a radioactive product. The patient is imaged using a gamma camera. Images are produced of the distribution of the radioactive substance within different organs and systems. This can be compared with normal distribution to diagnose if a medical condition is present and assess its extent or severity.

A multidisciplinary healthcare team, comprising medical staff, hospital physicists, radiopharmacists, medical technologists, radiography practitioners, nurses and clerical support staff, delivers the nuclear medicine service. Nuclear medicine departments can be autonomous or part of a diagnostic imaging department.

Key terms

Radionuclide: a species of atom whose nucleus disintegrates by the emission of alpha particles, beta particles or gamma radiation.

Radiopharmaceutical: a medicinal product dependent for all or part of its action on a radioactive ingredient. It may be used for diagnostic, therapeutic or research purposes.²

Half-life: the time taken for the radioactivity to decay to half its original value. In the body this depends on the physical properties of the radionuclide and how quickly it is excreted from the body.

Gamma camera: the instrument used to detect the radiation and produce images.

Technetium: the most common radionuclide used in medical imaging.

RADIONUCLIDES USED IN MEDICAL IMAGING

TECHNETIUM

Technetium-99m (abbreviated to ^99mTc) is the most common radionuclide used in medical imaging. ^99mTc is attached to a pharmaceutical to produce a radiopharmaceutical, which is normally injected intravenously into the body. ^99mTc is used because:

a) It has a half-life of 6 hours.

• If a radionuclide had a very short half-life, such as 30 minutes, it would be very difficult to transport it from its site of manufacture in time to use it. If it had a long half-life, such as 30 years, the dose to the patients would be too great because their bodies would be receiving a radiation dose for the rest of their lives. The radiation dose to the patient to achieve a diagnostic medical image should be ‘as low as reasonably practicable’ (ALARP). A very short half-life would mean that the physiological uptake of the radiopharmaceutical could not be observed because it can take several hours for physiological uptake to occur in some systems. It is easy to deal with articles and waste contaminated with ^99mTc because it has a short half-life. After a few days of storage the radiation dose is low enough to be considered insignificant and therefore articles can be disposed of or used again in the department.

b) It is a pure gamma emitter.

• Gamma emitters are part of the electromagnetic spectrum. This means that they do not damage cells as much as alpha and beta particles, which are charged. Radioactive decay occurs when an element has an unstable arrangement of protons or neutrons and transforms into a stable element.³ Most elements emit alpha or beta particles as well as gamma radiation. The gamma radiation emission is normally instantaneous, but with some elements the atom stays in an excited state for a prolonged period of time. These are called ‘metastable radionuclides’ and they emit radiation at a discrete energy level that is characteristic of the radionuclide and normally only emit gamma radiation. The ‘m’ in ^99mTc indicates that technetium is metastable.

c) It has a 140 keV energy level.

• The energy level characteristic of ^99mTc is 140 kiloelectron volts (keV). This means that the energy is high enough to go through the patient and interact with the gamma camera. If it were too low, it would be attenuated in the patient and would not be able to interact with the camera. An energy level of 140 keV is low enough to be attenuated by the distance of a normal gamma camera room and therefore the practitioner will not be over-irradiated by the radionuclide.

d) It is cheap and easy to produce.

• ^99mTc is produced in the morning, before work begins in the hospital, by passing saline over a column containing the radioactive element molybdenum in a radiopharmacy (Fig. 16.1). The molybdenum is contained in a radioactive generator. As the saline passes over the column of molybdenum the saline takes off the daughter product technetium, forming a radioactive saline. The parent molybdenum stays on the column.

• The radioactive saline is then tested to make sure it has not been contaminated by the parent because this would be dangerous to the patient due to the fact that molybdenum has a half-life of 67 hours and emits beta radiation. The ^99mTc is produced in sterile radiopharmacy (Fig. 16.2).

e) It binds easily to pharmaceuticals.

• ^99mTc in radioactive saline is added to freeze-dried pharmaceutical kits in the radiopharmacy (Fig. 16.3). The kits take about 10 minutes to bind to the radionuclide and then they are ready to sub dispense, so they can be injected into patients. Some kits have to be boiled before they bind. The radiopharmaceuticals are tested for chemical binding and sterility.

f) They are non-toxic.

• Unlike radiographic contrast media, technetium does not contain iodine and therefore reactions are very rare. Technetium radiopharmaceuticals contain microscopic amounts of pharmaceuticals and therefore allergic reactions do not normally occur.

Figure 16.1 Molybdenum generator.

Figure 16.2 Radiopharmacy.

Figure 16.3 Freeze dried radiopharmaceutical kit.

OTHER RADIONUCLIDES USED IN DIAGNOSTIC IMAGING

^81mKrypton

Krypton is used for scanning the lungs as it can show the ventilation. It is metastable like technetium and also a pure gamma emitter. It has an energy level of 190 keV, but has a half-life of 13 seconds. This means that the radionuclide has to be produced and then administered directly to the patient. In this case the patient inhales the radionuclide. Air is passed over a column of rubidium, which results in krypton being produced.

²⁰¹Thallium

Thallium is used for imaging the heart muscle (myocardium). It has a half-life of 73 h and has a principal energy level of 81 keV. It is not an ideal imaging agent due to the multiple energy levels and can be toxic in large amounts.

¹¹¹Indium

Indium is used for scanning a patient with infections. It is attached to the patient’s white blood cells. Indium has energy levels of 171 and 245 keV and a half-life of 2.8 days.⁴

EQUIPMENT

The instrument used to detect the radiation and produce images is a gamma camera. An image can be formed from the information gathered by the camera and displayed in either a static or whole body form (planar images) or dynamic mode (related to time; e.g. renal). Modern imaging systems can also create images in three dimensions (single photon emission computed tomography, SPECT), similar to those observed in computed tomography (CT) or magnetic resonance imaging (MRI).

The basic design of a gamma camera has not significantly changed for over 40 years and the use of devices such as sodium iodine crystals and photomultiplier tubes are the main reasons why nuclear medicine images have such low resolution in comparison to CT. However, technology advancements may see the development and production of solid-state gamma cameras in the future. The modern gamma camera consists of a large detector (or two detectors in dual head systems, Fig. 16.4), which is positioned as close to the patient as possible during examinations.

Figure 16.4 Modern dual head gamma camera system with an imaging couch and collimator cart.

Other features of a modern gamma camera system include an imaging couch, which is curved for patient comfort, a gantry for the detector heads to manoeuvre and a positioning monitor. The gamma camera is linked to a computer system which reflects the relative uptake of radiopharmaceutical tracer within the patient in the form of a visual image.

Many nuclear medicine departments will utilise one gamma camera to undertake a range of examinations. Some larger departments may employ a dual and a single head gamma camera to perform clinical examinations. Dual head gamma camera systems allow the operator to perform certain examinations (e.g. whole body bone scans) quicker than single head units, which is particularly useful for patients who may be in considerable discomfort.

The detector unit comprises a number of components, which enables the visualisation of radiopharmaceutical uptake within the patient. The gamma camera is a robust piece of medical imaging equipment; however, there is a requirement to ensure the working temperature of the examination room is kept constant and extreme fluctuations in temperature are avoided as this may have an impact upon the quality of the images produced.

The basic components of a modern gamma camera detector unit are:

• photomultiplier tubes

• shielding and cooling devices

• remote operating monitor.

Collimator

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