ABSNM NMPI key terms

A vacuum tube that converts light photons from a scintillator into an electrical signal and amplifies it through a series of dynodes. It provides high gain and fast response for radiation detection and gamma camera imaging.

A material that absorbs ionizing radiation and emits visible light photons in proportion to the energy deposited. Common examples include NaI(Tl) for gamma cameras and LSO for PET.

An electronic circuit that sorts detector pulses by amplitude into energy bins, allowing selection of a specific energy window. It is used to discriminate between photopeak events and scattered radiation.

The ability of a detector to distinguish between radiation of different energies, usually expressed as the full width at half maximum (FWHM) of a photopeak divided by its centroid energy, given as a percentage.

The time interval after a detected event during which the detector or processing electronics cannot record another event. It causes count loss at high counting rates and can be paralyzable or nonparalyzable.

A dead time model in which every event, including those occurring during the dead period, extends the dead time. At very high count rates, the observed count rate can decrease as the true rate increases.

A dead time model in which events occurring during the dead period do not extend it. The observed count rate approaches a maximum asymptote as the true count rate increases.

The peak in a gamma-ray spectrum corresponding to full absorption of the incident photon energy in the detector. It is used for energy window setting and for quantifying radionuclide uptake.

Interaction in which a gamma photon transfers part of its energy to an electron and is deflected, producing a scattered photon with lower energy. Scattered photons can degrade image contrast if detected.

A selected range of pulse heights (energies) accepted by the pulse height analyzer. It is typically centered on the photopeak, such as a 20% window around 140 keV for technetium-99m.

The degree to which a gamma camera produces a constant count rate across the useful field of view when exposed to a uniform flood source. It is expressed as integral and differential uniformity.

A measure of the maximum and minimum pixel counts across the entire useful field of view, expressed as a percentage. It reflects the overall variation in camera response.

A measure of local uniformity that compares adjacent pixels or small regions across the field of view. It is more sensitive to small localized nonuniformities than integral uniformity.

A uniform source of radioactivity used to assess gamma camera uniformity and sensitivity. It may be a liquid-filled flood phantom or a sheet source, and images are acquired with and without collimators.

The ability of an imaging system to distinguish two closely spaced point sources as separate. It is often expressed as full width at half maximum (FWHM) of a line spread function in millimeters.

The spatial resolution of a gamma camera without a collimator, determined by the detector and electronics. It is measured using a point or line source and is typically a few millimeters FWHM.

A device made of lead or tungsten with holes or slits that restricts the direction of gamma rays reaching the detector, thereby forming an image. Its geometry determines sensitivity and spatial resolution.

The passage of gamma rays through the septa (walls) between collimator holes rather than through the holes. It degrades image contrast and increases with higher-energy radionuclides and thinner septa.

The number of counts recorded per unit of radioactivity in the source, often expressed as counts per minute per microcurie (cpm/µCi). It depends on collimator geometry and detector efficiency.

Detection of gamma rays that have undergone Compton interaction in the patient or surrounding material and lost energy. Scattered events add background counts and reduce image contrast.

A processing method that estimates and subtracts scattered counts from an image to improve contrast and quantitation. Common approaches include energy-window-based methods and deconvolution.

A method that compensates for gamma-ray absorption and scattering within the patient, improving image uniformity and quantitative accuracy. It is commonly performed in SPECT/CT and PET/CT using transmission data.

An imaging procedure in which an external radioactive source or X-ray tube is used to measure attenuation through the patient. The data are used to create an attenuation map for correction.

A semiquantitative measure of radiotracer uptake in a region of interest, normalized to injected dose and patient body size. It is commonly used in PET oncology to assess lesion metabolism.

A defined area on an image used for quantitative analysis, such as measuring counts or SUV. The ROI should be placed consistently to ensure reproducible results.

The count rate profile obtained by imaging a thin line source, used to measure spatial resolution. Its full width at half maximum (FWHM) indicates the system's blurring.

A measure of how well an imaging system preserves contrast at different spatial frequencies. It is derived from the line spread function and describes spatial resolution performance.

A grainy appearance in images caused by statistical fluctuations in the number of detected photons. It becomes more noticeable with lower counts and can be reduced by increasing acquisition time or dose.

The number of detected events per unit time, usually expressed in counts per second or counts per minute. It is affected by source activity, detector efficiency, and dead time.

A PET technique that measures the difference in arrival times of annihilation photons at opposing detectors to localize the annihilation event along the line of response. It improves image signal-to-noise ratio.

An imaging modality that detects pairs of 511 keV annihilation photons emitted from positron-emitting radionuclides. Coincidence detection localizes the annihilation event along a line of response.

The straight line connecting two detectors that record a coincidence event in PET. The annihilation event is assumed to have occurred somewhere along this line.

The process of identifying two nearly simultaneous detector events in PET as originating from a single annihilation. A timing window is used to accept true coincidences and reject random events.

A coincidence event in which two unrelated photons from different annihilations are detected within the timing window. It adds background noise and is reduced by shorter timing windows and lower count rates.

A tomographic imaging technique that uses a rotating gamma camera to acquire multiple planar projections and reconstructs cross-sectional images. It typically uses single-photon-emitting radionuclides such as technetium-99m.

An imaging device that detects gamma rays using a large scintillation crystal, photomultiplier tubes, and a collimator to produce planar images of radionuclide distribution. It is the core instrument for conventional nuclear medicine.

A set of scheduled tests and measurements performed to ensure that imaging equipment operates within specified performance limits. QC includes uniformity, resolution, sensitivity, and energy peaking checks.

A series of measurements performed after installation of equipment to verify that it meets manufacturer specifications and regulatory requirements before clinical use. It establishes baseline performance values for future QC.

Adherence to laws, regulations, and standards governing the use of radioactive materials and medical imaging equipment. It includes licensing, radiation safety, and documentation requirements.