Nuclear medicine

Chapter 16 Nuclear medicine





KEY POINTS












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:



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.










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.



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:







Feb 20, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Nuclear medicine
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