NMAA key terms

An unstable form of an element that emits radiation as its nucleus decays toward stability. In nuclear medicine it is the source of the emitted photons or particles used for imaging or therapy.

A radioactive drug combining a radionuclide with a carrier molecule that directs it to a target tissue. Its biodistribution determines what an image shows and which organs receive radiation dose.

The time required for half of the radioactive atoms in a sample to decay. Physical half-life governs dose preparation and shelf life, while biologic half-life governs how quickly activity leaves the body.

The combined result of physical decay and biologic clearance, calculated as the product of physical and biologic half-lives divided by their sum. It predicts how long activity actually persists in a patient's tissue.

The time needed for the body to eliminate half of a substance through excretion or metabolism, independent of radioactive decay. It shapes organ residence time and therefore absorbed dose estimates.

The specific nuclear transformation a radionuclide undergoes, such as beta-minus, beta-plus, alpha, or isomeric transition. The mode determines which emissions are available for imaging or therapy and what shielding is needed.

Decay of a metastable nuclear state to a lower energy state by emitting a gamma photon, often with internal conversion. Technetium-99m decays this way, producing the 140 keV gamma rays used in most gamma camera imaging.

A nuclear energy level that persists measurably before decaying, denoted by the letter m after the mass number. It allows a radionuclide such as Tc-99m to be shipped and used as an imaging source.

Beta-plus decay in which a proton converts to a neutron and a positron is ejected. The positron annihilates with an electron to produce two 511 keV photons traveling in nearly opposite directions.

The interaction in which a positron and an electron combine and convert their mass into two photons of 511 keV each, emitted about 180 degrees apart. PET detectors rely on this geometry for coincidence detection.

A system that produces a short-lived radionuclide from the decay of a longer-lived parent held on a column. The Mo-99/Tc-99m generator supplies technetium eluate to most hospital nuclear medicine departments.

The process of drawing saline through a generator column to remove the accumulated daughter radionuclide. Elution efficiency and elapsed time since the previous elution determine the activity yield obtained.

The measured activity of daughter radionuclide obtained from a generator divided by the activity expected at that time, expressed as a percentage. A declining value can indicate column problems or breakthrough.

The fraction of total activity in a preparation that comes from the intended radionuclide rather than contaminants. For technetium eluate, molybdenum-99 breakthrough must be measured and kept within regulatory limits.

The proportion of activity present in the desired chemical form rather than as free radionuclide or other labeled species. It is checked by chromatography because impurities alter biodistribution and image quality.

A vial of nonradioactive reagents, including the targeting molecule and reducing agents, to which pertechnetate is added to form a radiopharmaceutical. Labeling yield depends on proper kit preparation and storage.

The free Tc-99m ion in saline, chemically similar to iodide, which is taken up by thyroid, salivary glands, and stomach. It is used directly for some scans but is also the starting material for labeling kits.

The pattern of where and how quickly a radiopharmaceutical accumulates in the body over time. It determines normal organ uptake, target-to-background contrast, and the timing of image acquisition.

The fraction of administered activity that localizes in a tissue of interest, often reported as a percentage of the injected dose. It reflects blood flow, transporter expression, and metabolism in that tissue.

The activity concentration in a lesion or organ of interest divided by that in surrounding tissue. Higher ratios improve lesion detectability, and the ratio usually changes with time after injection.

The average time that activity remains in a source organ, obtained from time-activity data. It is a required input for absorbed dose calculations in radionuclide therapy and dosimetry.

The energy deposited per unit mass of tissue by ionizing radiation, measured in gray or rad. It links administered activity to biologic risk and is the basis for radiation protection limits.

Absorbed dose averaged over a tissue and weighted for the relative harm of the radiation type, measured in sievert or rem. It allows comparison of risk across different radiation exposures.

A whole-body risk index that sums equivalent doses in organs weighted by their radiation sensitivity, expressed in sievert. It is used to compare the stochastic risk of different nuclear medicine procedures.

The principle of keeping radiation exposure as low as reasonably achievable while still accomplishing the clinical purpose. It drives time, distance, and shielding practices and optimization of administered activity.

A well-type ionization chamber used to measure the activity of a radiopharmaceutical before administration. It must be checked for accuracy, linearity, and geometry effects on a scheduled quality control basis.

A calculation that adjusts an activity value from one time to another using the radionuclide's half-life. It is used to determine how much activity remains at injection time or how much to draw now.

An imaging device that detects gamma rays with a scintillation crystal and converts light flashes into position signals through photomultiplier tubes. It produces planar images of radiotracer distribution.

A lead plate with holes placed in front of the camera crystal that restricts incoming photons to near-parallel paths. Its hole size and length trade spatial resolution against sensitivity.

The peak in a gamma spectrum produced by full-energy absorption of incident photons in the detector. Its position identifies the radionuclide and is used to set the camera energy window.

A selected range of pulse heights accepted by the camera, usually centered on the photopeak. A narrow window reduces scatter but may lose counts if the peak drifts.

Photons that change direction and lose energy after interacting in the patient or detector. Scattered events add background and blur contrast, so cameras use energy windows and correction methods to limit them.

The smallest separation at which two adjacent sources can still be distinguished in an image. It is expressed as full width at half maximum and worsens with distance from the collimator.

The number of detected counts per unit of activity in the source, reflecting how efficiently the system captures emitted photons. Higher sensitivity shortens scans but usually costs resolution.

The difference in displayed signal between a lesion and its surroundings relative to background. It depends on target-to-background uptake, scatter rejection, and correct window settings.

Single photon emission computed tomography, which rotates a gamma camera around the patient to reconstruct three-dimensional distributions of a gamma-emitting tracer. It improves lesion localization over planar imaging.

Positron emission tomography, which detects annihilation photon pairs in coincidence to map positron-emitting tracers with high sensitivity. It requires cyclotron-produced radionuclides and correction for attenuation.

Electronic acceptance of two detector events within a short time window as a single annihilation event. It localizes the source along a line of response and rejects most single unrelated photons.

Compensation for photon loss as radiation passes through tissue before reaching the detector. Without it, deep structures appear artificially low in activity, especially in PET and cardiac SPECT.

The mathematical process that converts projection data into cross-sectional images, using filtered backprojection or iterative methods. Iterative reconstruction with correction modeling generally improves image quality.

Scheduled testing that verifies equipment and radiopharmaceuticals meet performance specifications before clinical use. Results are documented and out-of-limit findings trigger corrective action.