ABR Nuclear Radiology key terms

An interaction in which an incident photon transfers all its energy to an inner-shell electron, ejecting it; the resulting vacancy produces characteristic x-rays. It dominates at low photon energies and in high-atomic-number materials such as lead shields and bone.

An interaction in which a photon scatters off an electron, losing part of its energy and changing direction. The recoil electron and scattered photon share the remaining energy, and many such events create a broad scatter continuum that degrades image contrast.

The peak in a gamma-ray energy spectrum produced when the full energy of an incident photon is deposited in the detector, usually through photoelectric absorption. Its position identifies the radionuclide, and its width reflects the detector's energy resolution.

A detector's ability to distinguish photons with closely spaced energies, usually expressed as the full width at half maximum of a photopeak divided by its centroid energy. Better resolution separates photopeaks and limits scattered photons in the energy window.

A scintillation camera that uses a large sodium iodide crystal, a light guide, and an array of photomultiplier tubes with position logic to localize gamma interactions in two dimensions, generating planar images or tomographic projections.

A lead or tungsten plate pierced by many parallel channels that admits mainly photons traveling perpendicular to the detector face. It preserves image size while reducing sensitivity, and channel length and septal thickness determine spatial resolution and septal penetration.

The brief interval after a detector event during which the system cannot record another pulse, causing count losses as activity rises. It makes count rate nonlinear and can distort quantitative uptake measurements in high-activity studies.

An electronic circuit that sorts detector pulses by amplitude into energy windows, allowing selection of a chosen photopeak and rejection of scattered photons. It underlies radionuclide imaging and gamma-ray spectroscopy.

The identification of two annihilation photons arriving within a narrow timing window at opposing detectors, which defines a PET event. Accurate timing rejects random coincidences and localizes the annihilation along the line of response.

PET that measures the small arrival-time difference between the two annihilation photons to estimate how far along the line of response the annihilation occurred, improving signal-to-noise ratio, especially in large patients.

A semiquantitative PET measure that normalizes tissue activity concentration to injected dose and body size, usually body weight. It allows lesion comparison but varies with acquisition timing, reconstruction, and biologic factors.

Underestimation of activity concentration in objects smaller than about two to three times the scanner resolution because counts from the object mix with background and spill outside the region of interest.

Correction of PET or SPECT emission data using a CT transmission image converted from Hounsfield units to attenuation coefficients at the emission photon energy, improving quantitative accuracy and reducing artifacts caused by tissue attenuation.

A device in which a long-lived parent radionuclide decays to a shorter-lived daughter that is eluted for clinical use. The molybdenum-99/technetium-99m generator requires periodic elution and checks of radionuclidic and radiochemical purity.

The proportion of total radioactivity present in the desired chemical form, usually measured by chromatography. Impurities can change biodistribution and dosimetry, so purity is verified before administering compounded radiopharmaceuticals.

The time for radioactivity in a biologic system to fall by half through combined physical decay and biological clearance. It is calculated from the physical and biological half-lives and determines imaging timing and absorbed dose.

The radiation protection principle of keeping exposures as low as reasonably achievable, accounting for economic and social factors, by using shielding, distance, time, and optimized imaging protocols.

A radiation protection quantity that sums equivalent doses to organs and tissues weighted by their radiation sensitivity, expressed in sieverts, permitting comparison of stochastic risk across different exposure types.

In the Medical Internal Radiation Dose formalism, a coefficient giving absorbed dose to a target organ per unit cumulated activity in a source organ, incorporating radionuclide decay data and patient geometry.

A benchmark activity or dose for a defined imaging procedure, derived from practice surveys, used to flag unusually high exposures and encourage optimization rather than to define a regulatory limit.

A five-point visual scale that compares FDG uptake in a lymphoma lesion with mediastinal blood pool and liver background, used to classify metabolic response after therapy.

A standardized framework for quantitative FDG PET response assessment that uses liver uptake as a reference, defines measurable target lesions, and classifies response by changes in peak standardized uptake value.

PET imaging with ligands targeting prostate-specific membrane antigen, usually labeled with gallium-68 or fluorine-18, used to localize prostate cancer recurrence and metastases with high sensitivity.

PET with gallium-68 DOTATATE or similar ligands that bind somatostatin receptor subtype 2, used to detect and stage neuroendocrine tumors and to select patients for peptide receptor radionuclide therapy.

SPECT imaging with technetium-99m or thallium-201 tracers during stress and rest to assess regional myocardial blood flow, identifying ischemia and scar through reversible or fixed perfusion defects.

Acquisition of myocardial perfusion data synchronized to the electrocardiogram, permitting assessment of left ventricular ejection fraction, wall motion, and wall thickening in addition to perfusion.

Scintigraphy with meta-iodobenzylguanidine, an analog taken up by sympathetic nerve terminals, used to evaluate pheochromocytoma, paraganglioma, and cardiac sympathetic innervation.

Imaging with technetium-99m sestamibi, which accumulates in parathyroid adenomas and thyroid tissue, often using delayed or subtraction views to localize hyperfunctioning parathyroid glands before surgery.

Cholescintigraphy with technetium-99m iminodiacetic acid agents that hepatocytes take up and excrete into bile, used to assess biliary obstruction, bile leaks, and acute cholecystitis.

A renal scan with a tubular agent such as technetium-99m MAG3 combined with a loop diuretic to distinguish obstructed from dilated but unobstructed collecting systems by comparing drainage before and after diuresis.

A lung study comparing inhaled radiotracer distribution with intravenous perfusion tracer distribution. Mismatched defects suggest pulmonary embolism, whereas matched defects suggest parenchymal lung disease.

Skeletal scintigraphy comprising flow, blood-pool, and delayed phases after injection of technetium-99m diphosphonate, used to distinguish hyperemia and infection or tumor from ordinary degenerative uptake.

Scintigraphy using indium-111 or technetium-99m labeled white blood cells that migrate to sites of infection or inflammation, useful when anatomic imaging is inconclusive.

A dopamine transporter imaging agent for SPECT that shows reduced striatal uptake in Parkinsonian syndromes, helping distinguish neurodegenerative parkinsonism from essential tremor and drug-induced parkinsonism.

Oral radioactive iodine used to ablate residual thyroid tissue or treat differentiated thyroid cancer and hyperthyroidism, relying on sodium-iodide symporter uptake and requiring radiation safety counseling.

Treatment with radiolabeled somatostatin analogs such as lutetium-177 DOTATATE that bind tumor somatostatin receptors, delivering beta radiation to neuroendocrine tumors while limiting normal tissue dose.

Catheter-directed infusion of yttrium-90 microspheres into hepatic arteries supplying tumors, delivering high local beta dose; pretreatment technetium-99m macroaggregated albumin mapping assesses lung shunt and extrahepatic flow.