Nuclear medicine is a branch of medicine and medical imaging that uses the nuclear properties of matter in diagnosis and therapy. Many procedures in nuclear medicine use radionuclides, or pharmaceuticals that have been labeled with radionuclides (radiopharmaceuticals). In diagnosis, radioactive substances are administered to patients and the radiation emitted is measured. The majority of these diagnostic tests involve the formation of an image using a gamma camera. Imaging may also be referred to as radionuclide imaging or nuclear scintigraphy. Other diagnostic tests use probes to acquire measurements from parts of the body, or counters for the measurement of samples taken from the patient. In therapy, radionuclides are administered to treat disease or provide palliative pain relief. For example, administration of Iodine-131 is often used for the treatment of thyrotoxicosis and thyroid cancer.
Nuclear medicine differ from most other imaging modalities in that the tests primarily show the physiological function of the system being investigated as opposed to the anatomy. In some centres, the nuclear medicine images can be superimposed on images from modalities such as CT or MRI to highlight which part of the body the radiopharmaceutical is concentrated in. This practice is often referred to as image fusion or co-registration.
Nuclear medicine diagnostic tests are usually provided by a dedicated department within a hospital and may include facilities for the preparation of radiopharmaceuticals. The specific name of a department can vary from hospital to hospital, with the most common names being the nuclear medicine department and the radioisotope department.
Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. The radionuclide introduced into the body is often chemically bound to a complex that acts characteristically within the body; this is commonly known as a tracer. In the presence of disease, a tracer will often be distributed around the body and/or processed differently. 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 via the hydroxyapatite for imaging. Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a 'hot-spot' which is a focal increase in radio-accumulation, or a general increase in radio-accumulation throughout the physiological system. 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 types of tests can be split into two broad groups: in-vivo and in-vitro:
A typical nuclear medicine study involves administration of a radionuclide into the body by injection in liquid or aggregate form, ingestion while combined with food, inhalation in gaseous form or, rarely, injection of a radionuclide that has undergone micro-encapsulation. Some specialist studies require the labeling of a patient's own cells with a radionuclide (leukocyte scintigraphy and red cell scintigraphy). 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 or fusion processes in nuclear reactors, which produce radioisotopes with longer half-lives, or cyclotrons, which produce radioisotopes with shorter half-lives, or take advantage of natural decay processes in dedicated generators, i.e. Molybdenum/Technetium or Strontium/Rubidium.
The most commonly used liquid radionuclides are:
The most commonly used gaseous/aerosol radionuclides are:
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 thallium-doped sodium iodide NaI(Tl) scintillation crystal, coupled with an imaging sub-system such as an array of photomultiplier tubes and associated electronics. Solid-state gamma-ray detectors are available, but are not yet commonplace. Gamma-cameras employ lead or tungsten collimators to form an image on the crystal, accepting photons arriving perpendicular to the camera face, and rejecting off-axis photons which would degrade the desired image.
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 detects the X and Y position of each gamma-ray event, using these coordinates to place a pixel in an image matrix to build a recognisable image. The units of a raw nuclear medicine image are 'counts' or 'kilocounts', referring to the number of gamma-ray events detected. In nuclear medicine, the value of an image pixel is the integral of gamma-ray events in that pixel position over time. That is, the pixel appears brighter as more counts are detected in that position. 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. Activity closer to the camera face will produce more information in the image than activity located deeper in the body, however, because of attenuation by tissues between the radionuclide event and the camera face. Tomographic imaging applies similar principles, taking multiple planar images from different angles and then refining them using a process known as filtered back projection generating three dimensional views of organs or areas of interest.
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, most often with a plus or minus ten percent window, of the energy spectrum of a particular radionuclide, thus ignoring 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 form a slice-stack, a three-dimensional representation of the distribution of radionuclide in the patient.
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,
A patient undergoing a nuclear medicine procedure will receive a radiation dose. Under present international guidelines it is assumed that any radiation dose, however small, presents a risk. The radiation doses delivered to a patient in a nuclear medicine investigation present a very small risk of inducing cancer. In this respect it is similar to the risk from X-ray investigations except that the dose is delivered internally rather than externally.
The radiation dose from a nuclear medicine investigation is expressed as an effective dose with units of sieverts (usually given in millisieverts, mSv). The effective dose resulting from an investigation is influenced by the amount of radioactivity administered in megabecquerels (MBq), the physical properties of the radiopharmaceutical used, its distribution in the body and its rate of clearance from the body.
Effective doses can range from 6 μSv (0.006 mSv) for a 3 MBq chromium-51 EDTA measurement of glomerular filtration rate to 37 mSv for a 150 MBq thallium-201 non-specific tumour imaging procedure. The common bone scan with 600 MBq of technetium-99m-MDP has an effective dose of 3 mSv (1).
It can be noted that more radiation is absorbed by the body during a single, short (1-2 hour), airplane flight than is taken in by the majority of nuclear medicine studies.
1. Notes for guidance on the clinical administration of radiopharmaceuticals and use of sealed radioactive sources. Administration of radioactive substances committee UK 1998.
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