Nuclear Medicine and Diagnostic Imaging (NMDI)
Hints and FAQs
This section is intended to provide hints and answers to some Frequently Asked Questions (FAQ) in relation with the activities of the International Atomic Energy Agency (IAEA) in Nuclear Medicine (NMS). For further questions and answers please contact NMS. We are open to suggestions and comments on these items, and any feedback is most welcome.
What is nuclear medicine?
Nuclear medicine can be broadly divided into two branches "in vitro" and "in vivo" procedures. There are numerous radioisotopic "in vitro" procedures for genotyping and molecular profiling applicable to clinical molecular biology. These procedures are becoming increasingly important in several clinical and pre-clinical conditions, from determining changes in cancer cells to drug resistance in malaria parasites. These techniques proved to be increasingly valuable in preventing catastrophic consequence of ineffective treatment.
The majority of nuclear medicine procedures are "in vivo" non-invasive procedures. After administration of the radiopharmaceutical typically by intravenous route (sometimes locally) to a patient, its distribution and localization provides functional or metabolic information. This helps doctors to make critical decisions based on objective information about the status and function of a particular organ or disease .The data is depicted with the aide of imaging systems called gamma-cameras (be they planar or SPECT systems) and transformed into images which allow visual determination and staging of the disease. One of the fastest growing techniques is , positron emission tomography (PET) that requires special instrumentations called PET tomographs. This technique allows clinicians to track organ function at a molecular level, therefore revealing intricate health changes earlier in individual patients than other diagnostic modalities.
The majority of nuclear medicine procedures are "in vivo" non-invasive procedures. After administration of the radiopharmaceutical (4) its distribution and localization provides functional or metabolic information The data is depicted with the aide of imaging systems called gamma-cameras which allow doctors to make critical decisions and staging of the disease. Radiology or X-rays provides anatomical information which when supplemented with functional imaging becomes indispensable tools for the diagnosis and treatment of a large number of benign and malignant disorders.
Positron emission tomography (PET)
One of the fastest growing techniques is, positron emission tomography (PET) that requires special instrumentations called PET tomographs. This technique allows clinicians to track organ function at a molecular level, therefore revealing intricate health changes earlier in individual patients than other diagnostic modalities.
Nuclear Medicine therapeutic applications
Nuclear medicine has also therapeutic applications and for over 50 years it has been successfully used for the treatment of thyrotoxicosis and malignant diseases. There is a growing field of application in oncology, namely in the treatment of liver cancer, lymphomas, neuroendocrine tumours as well as in the palliative treatment of metastatic bone pain.
The subprogramme has significantly contributed to education and training in the field of nuclear medicine through use of information and communication tools like Internet, tele-nuclear medicine and distance assisted training. Tele-medicine has become an essential component of modern health care. It has been shown to be efficient and cost-effective, and an organ for change and betterment in developing countries.
What is radioimmmunoasssay?
It is an extremely robust and one of the most sensitive techniques for quantitation of analytes in biological fluid or tissue extract at the range of pico to femto mole concentration using biologically produced binder known as antibody which reacts specifically with the analyte of interest in the presence of structurally similar radiolabelled molecule in a competitive manner for measuring small molecule or with the help of an excess second radiolabelled antibody to form a sandwich for the detection of large molecule. The bound radio signal is used to compute the concentration of the analyte in the biological fluid from a calibration curve.
What is a radionuclide generator?
A radionuclide generator is a device which permits ready separation of a daughter radionuclide from its parent. In generator systems of practical importance the parent has a relatively long half-life compared with the daughter, and the device (often referred as a "cow") permits repeated elutions ("milkings") at suitable intervals. Generators make possible the routine use of certain short-lived radionuclides at locations remote from centres of radionuclide production. The example of outstanding importance is that the 99Mo 99mTc generator. 99mTc is widely used in diagnostic medicine, as it emits as gamma photon of ideal energy (140 keV) for imaging, and its half-life (6.02 h) and freedom of beta radiation lead to low radiological doses to the patient.
What is a radiopharmaceutical?
A sterile, pyrogen free radionuclide or radioactively tagged compound administered to a patient for diagnostic or therapeutic purposes. A radiopharmaceutical has no pharmacological effect because of the small amount of material administered.
What is SPECT?
Single Photon Emission Computerized Tomography is a technique that uses a computer for tomographic reconstruction (in a variety of planes, transaxial, coronal, sagital) of the distribution of a single photon gamma emitting radionuclide detected by a rotating gamma camera (the widely used Tc-99m, which emits single 140 kev gamma photons, is an example). Its essential goal is enhancement of the image detectability and the extraction of quantitative data from a true three dimensional distribution of structure (or radioactivity) in space. The title SPECT excludes positron emission tomography (PET) from discussion, as PET is based on the detection , by means of opposed detectors and coincidence counting techniques, of the two 511 kev photons which are simultaneously emitted in almost opposite directions by a positron emitting radionuclide. Any conventional (planar) imaging technique is restricted mainly to the visualization of objects in three dimensional space by two dimensional projections. Multiple views of an object from different angles are required to appreciate its three dimensional structure. These separate views are usually called projections. A complete set of projections potentially permits a complete reconstruction of object structure in three dimensions.
What is radioimmunotherapy?
The ability to treat certain types of tumours using Beta-emitting radiolabelled monoclonal antibodies directed against tumour antigens.
What is radionuclide therapy?
Therapy of diseases by the intracavitary, intravenous, oral, or other routes of administration of sealed and unsealed radiopharmaceuticals emitting electrons, gamma-rays, or X-rays.
What is radiopharmacy?
Laboratory preparation and dispensing of solutions labelled with radioisotopes for therapeutic and diagnostic purposes.
What is scintigraphy?
The process of obtaining an image or series of sequential images of the distribution of a radionuclide in tissues, organs, or body systems using a scintillation gamma camera,e.g. hepatic scintigraphy, renal scintigraphy etc. Also known as radionuclide scintigraphy.
What is scintillation?
Name given to the production of light flashes emitted by luminescent substances when excited by high-energy radiation. The flashes can in turn liberate photoelectrons from photosensitive substances (for instance, caesium/antimony phosphors or some type of inorganic salt crystal such as NaI) (Tl activated); the photoelectrons are amplified by means of a photomultiplier tube before being converted into current pulses. The pulse height depends on the energy or the original gamma or corpuscular radiation, and the pulses can thus be sorted by means of a discriminator (pulse height analyzer). By using different discriminator channels the different nuclides in a mixture of isotopes can be determined either successively or simultaneously.
What is a scintillation detector?
The ionizing radiation detection system used in gamma cameras, rectilinear scanners, and gamma counters. It consists of a scintillator (usually a sodium iodide crystal, thallium activated), a photomultiplier (PM) tube, and supporting electronics. Scintillators glow when exposed to X-rays and small flashes occur in response to gamma rays. Thus an electric pulse can be generated by the photomultiplier tube for each gamma-ray interaction. This system is employed in the gamma camera and rectilinear scanner, to produce a map of radionuclide distribution whithin the body. In a well-scintillation counter the count rate indicates the quantity of a radionuclide and the intensity of the scintillation indicates the energy of the gamma-rays.
What is solid phase radioimmunoassay?
A modification of radioimmunoassay in which an antibody is absorbed onto solid particles or tubes.
What is a gamma camera?
An imaging apparatus used to visualize the distribution of radionuclides within the body. The majority of gamma cameras in clinical use operate on the principle devised originally by H.O. Anger at the Donner Laboratory in Berkeley in 1956. Radiation emanating from the patient is detected by a single, large, circular NaI (Tl) scintillation crystal. An array of (37, 61 or 91) photomultiplier tubes detects the light quanta emitted by the crystal and converts their energies into electrical pulses. Associated electronic circuitry determines the x, y coordinates of each scintillation event, the outputs of all the tubes being summed to provide a z pulse, the amplitude of which corresponds to the total energy of the scintillation event. The distribution of radioactivity is usually displayed on an oscilloscope. For quantitative analysis, the gamma camera is connected to a dedicated microcomputer, radioactive distributions being displayed either on a monochrome TC monitor as a gray scale range of tomes or on a color TC monitor as a color scale.
How are radionuclide based in vitro molecular techniques used for the diagnosis of diseases?
Molecular diagnosis is based on the recognition of short stretches of nucleic acids which uniquely identify a pathogenic organism or an abnormal piece of genetic information. Techniques such the polymerase chase reaction (PCR) which amplify nucleic acids a million fold are highly sensitive. Hybridisation of the PCR products using radioisotope labelled nucleic acid probes increases the specificity of the diagnosis, is useful for the quality assurance of the results and can further increase the sensitivity of the diagnosis. The ability to increase the sensitivity of the method by several means translates into an obvious benefit for the patient which is particularly important for young children in that a very small sample e.g. finger prick amounts of blood are sufficient. The million fold amplification also allows for the detection of pathogens which are present in very low numbers in clinical specimens for diseases such as Tuberculosis and Chagas. Additionally it allows for the much earlier diagnosis which has implications of reduced morbidity and mortality.
What are the advantages of isotopic methods?
Isotopic methods, in practice, have an unequalled degree of sensitivity and specificity. The techniques of labelling and detection are simple, robust and well established. Most non-radioactive methods make use of labels consisting of chemical groups that are often large enough to interfere with the process being investigated. Isotopic methods are quantifiable which is not always the case with non-isotopic ones. In detecting point mutations, e.g. using SSCP, there is better discrimination between the wild type and the mutant using isotopes.