Nuclear medicine

Information about Nuclear medicine

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Shown above is the bone scintigraphy of a young woman.
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 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 differs 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 testing

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:
  • In-vivo tests are measurements directly involving the patient. By far the most common are gamma camera imaging investigations, though non-imaging probes are also used to measure the levels of radioactivity within a patient.
  • In-vitro tests are measurements of samples taken from the patient (e.g. blood, urine, breath).

Types of studies

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:

Imaging equipment

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Diagrammatic cross section of gamma camera detector
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[1], 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.

Analysis

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,

Radiation dose

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).

See also

References

1. Notes for guidance on the clinical administration of radiopharmaceuticals and use of sealed radioactive sources. Administration of radioactive substances committee UK 1998.

External links

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Medicine is the science and "" of maintaining and/or restoring human health through the study, diagnosis, and treatment of patients. The term is derived from the Latin ars medicina meaning the art of healing.
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See also: Radiology, Radiography
Medical imaging refers to the techniques and processes used to create images of the human body (or parts thereof) for clinical purposes (medical procedures seeking to reveal, diagnose or examine disease) or medical science
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A pharmaceutical company, or drug company, is a commercial business whose focus is to research, develop, market and/or distribute drugs, most commonly in the context of healthcare. They can deal in generic and/or brand medications.
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Radiopharmacology is the study and preparation of radiopharmaceuticals, which are radioactive pharmaceuticals. Radiopharmaceuticals are used in the field of nuclear medicine as tracers in the diagnosis and treatment of many diseases.
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patient is any person who receives medical attention, care, or treatment. The person is most often ill or injured and in need of treatment by a physician or other medical professional.
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Ionizing radiation is energetic particles or waves that have the potential to ionize an atom or molecule through atomic interactions. It is a function of the energy of the individual particles or waves, and not a function of the number of particles or waves present.
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A gamma camera is a device used in nuclear medical imaging also known as nuclear medicine, to view and analyse images of the human body of the distribution of medically injected, inhaled, or ingested gamma ray emitting radionuclides.
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Palliative care (from Latin palliare, to cloak) is any form of medical care or treatment that concentrates on reducing the severity of disease symptoms, rather than providing a cure.
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Iodine-131 (131I), also called radioiodine, is a radioisotope of iodine.

131I decays with a half-life of 8.0197 days with beta and gamma emissions.
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Hyperthyroidism
Classification & external resources

Triiodothyronine (T3, pictured) and thyroxine (T4) are both forms of thyroid hormone.
ICD-10 E 05.
ICD-9 242 , 775.
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Thyroid cancer
Classification & external resources

ICD-10 C73
ICD-9 193

Thyroid cancer refers to any of four kinds of tumors of the thyroid gland: papillary, follicular, medullary and anaplastic.
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Physiology (from Greek: φυσις, physis, “nature, origin”; and λόγος, logos, "knowledge") is the study of the mechanical, physical, and biochemical functions of living organisms.
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Anatomy (from the Greek ἀνατομία anatomia, from ἀνατέμνειν
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Computed tomography (CT), originally known as computed axial tomography (CAT or CT scan) and body section roentgenography, is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the
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Magnetic resonance imaging (MRI), formerly referred to as magnetic resonance tomography (MRT) and, in scientific circles and as originally marketed by companies such as General Electric, nuclear magnetic resonance imaging (NMRI) or NMR zeugmatography imaging
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hospital is an institution for health care, often but not always providing for longer-term patient stays. Today, hospitals are usually funded by the state, health organizations (for profit or non-profit), health insurances or charities, including direct charitable donations.
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complex in chemistry usually is used to describe molecules or ensembles formed by the combination of ligands and metal ions. Originally, a complex implied a reversible association of molecules, atoms, or ions through weak chemical bonds.
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In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as
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Bone imaging is a study to visually detect bone abnormalities. Such imaging studies include magnetic resonance imaging (MRI), X-ray computed tomography (CT) and especially nuclear medicine.
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Hydroxylapatite, also often incorrectly called hydroxyapatite, is a mineral. It is a naturally occurring form of calcium apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6
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organ (Latin: organum, "instrument, tool") is a group of tissues that perform a specific function or group of functions. Usually there is a main tissue and sporadic tissues. The main tissue is the one that is unique for the specific organ.
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gland is an organ in an animal's body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).
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In vivo (Latin: (with)in the living) means that which takes place inside an organism. In science, in vivo refers to experimentation done in or on the living tissue of a whole, living organism as opposed to a partial or dead one.
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In vitro (Latin: (with) in the glass) refers to the technique of performing a given experiment in a test tube, or, generally, in a controlled environment outside a living organism. In vitro fertilization is a well-known example of this.
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A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy which is available to be imparted either to a newly-created radiation particle within the nucleus, or else to an atomic electron (see internal conversion) .
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Micro-encapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules with many useful properties. In a relatively simplistic form, a microcapsule is a small sphere with a uniform wall around it.
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White blood cells or leukocytes are cells of the immune system which defend the body against both infectious disease and foreign materials. Several different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone
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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 pharmaceuticals that have been labeled with radionuclides (radiopharmaceuticals).
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Red blood cells are the most common type of blood cell and the vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood.
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For the music band, see .
Gamma rays or gamma-ray (denoted as γ) are forms of electromagnetic radiation (EMR) or light emissions of a specific frequency produced from sub-atomic particle interaction, such as electron-positron annihilation and
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