Nuclear medicine is the branch of medicine that uses unsealed radioactive substances in diagnosis and therapy. It may also be referred to as radionuclide imaging or nuclear scintigraphy.
Within nuclear medicine there are a wide variety of investigations and therapies, all of which involve the administration of one or more radioactive substances to a patient. Nuclear medicine has applications in neurology, cardiology, oncology, endocrinology, lymphatics, urinary function, gastroenterology, pulmonology and other areas. One of the more common unsealed source radiotherapies is radioiodine administration.
Nuclear medicine diagnostic tests are provided by a dedicated department within a hospital. The specific name of the department can vary from hospital to hospital, with the commonest names being the nuclear medicine department and the radioisotope department.
Types of studies
A typical nuclear medicine study involves introduction of a radionuclide into the body via injection in liquid or aggregate form, inhalation in gaseous form or, rarely, injection of a radionuclide that has undergone microencapsulation. Most diagnostic radionuclides emit gamma rays, while the cell-damaging properties of beta particles are used in therapeutic applications. Refined radionuclides for use in nuclear medicine are derived from fission processes in nuclear reactors or cyclotrons.
The most commonly used liquid radionuclides are:
The most commonly used gaseous/aerosol radionuclides are:
- technetium-99m Technegas (http://jcsmr.anu.edu.au/technegas/home.html)®
- technetium-99m DTPA
Often, the radionuclide is administered directly, e.g. intravenously. Some specialist studies require the labeling of a patient's own cells with a radionuclide (lymphocyte scintigraphy and red cell scintigraphy).
The radiation emitted from the radionuclide inside the body is usually detected using a gamma camera. Traditionally, gamma-cameras have consisted of a gamma-ray detector, such as a single large sodium iodide scintillation crystal, coupled with an imaging sub-system such as an array of photo-multiplier tubes and associated electronics. Solid-state gamma-ray detectors are available (http://www.digirad.com), but are not yet commonplace. Gamma-cameras employ lead collimators to form an image of the radionuclide distribution in the body on the gamma-ray detector.
Gamma-camera performance is usually a balance of spatial resolution against sensitivity. A typical gamma-camera will have a resolution of 4 to 6 mm and will be able to capture several hundred thousand gamma-ray 'events' per second. The gamma-camera will detect the X an Y position of each gamma-ray event, and these coordinates will be used to build an image, as shown above. In nuclear medicine, the value of an image pixel is the integral of gamma-ray events in that pixel position over time. In non-tomographic images, the pixel can also be thought of as the line integral of radionuclide distribution of a perpendicular line extending from the pixel position through the body of the patient. The units of a raw nuclear medicine image is 'counts' or 'kilocounts', referring to the number of gamma-ray events detected.
Since each nuclear medicine radionuclide has a unique gamma-ray emission energy spectrum, and since the energy of a gamma-ray is detected in a gamma-camera by the brightness of the scintillation associated with an event, gamma-cameras employ energy 'windows' to gate or limit the imaging process to gamma-ray events of particular energies. An energy window is usually tailored to the peak of the energy spectrum of a particular radionuclide, and to ignore other gamma-rays that would otherwise contribute noise to the image. This allows noise caused by Compton scattering to be gated out.
The end result of the nuclear medicine imaging process is a "dataset" comprising one or more images. In multi-image datasets the array of images may represent a time sequence (ie. cine or movie) often called a "dynamic" dataset, a cardiac gated time sequence, or a spatial sequence where the gamma-camera is moved relative to the patient. SPECT (single photon emission computed tomography) is the process by which images acquired from a rotating gamma-camera are reconstructed to produce an image of a "slice" through the patient at a particular position. A collection of parallel slices for a slice-stack which is a three-dimensional (3D) representation of the distribution of radionuclide in the patient.
The radionuclide introduced into the body is often chemically bound to a complex that acts characterisically within the body; this is known as a tracer. For example, the ligand methylene-diphosphonate (MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone for imaging. Any increased physiological function will usually mean increased concentration of the tracer. This often results in the appearance of a 'hot-spot' (focal increase in radio-accumulation), or generally increased radio-accumulation. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a 'cold-spot'. Many tracer complexes have been developed in order to image or treat many different organs, glands, and physiological processes. The nuclear medicine computer may require millions of lines of source code to provide quantitative analysis packages for each of the specific imaging techniques available in nuclear medicine.
Radiation dose to a patient from a nuclear medicine study depends on the physical half-life of the radionuclide (the rate of radioactive decay) in conjunction with the initial level of radioactivity introduced, and the biological half-life of the radionuclide (the rate of excretion from the body). The level of radioactivity introduced to the body depends on the type of study being performed, but is typically within the range of 37 to 1110 MBq (1 to 30 mCi).