Chapter 16 Nuclear medicine
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.
RADIONUCLIDES USED IN MEDICAL IMAGING
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:
OTHER RADIONUCLIDES USED IN DIAGNOSTIC IMAGING
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.
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.
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.
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: