ABNM Nuclear Medicine key terms

An interaction in which an incident photon transfers all its energy to an inner-shell electron, ejecting it; dominant at low photon energies and high atomic number, producing characteristic x-rays.

An interaction in which a photon transfers part of its energy to an outer-shell electron and deflects at an angle; dominant at diagnostic energies and the main source of image-degrading scatter.

A vacuum device that converts light from a scintillation crystal into an electrical pulse and amplifies it through a series of dynodes, enabling position and energy information to be extracted.

A lead plate with many straight parallel channels that accepts only photons traveling perpendicular to the detector; hole diameter and length trade spatial resolution against sensitivity.

The ability of a detector system to distinguish photons of different energies, usually expressed as the percentage full width at half maximum of a photopeak; better resolution improves scatter rejection.

The interval after each detected event during which the system cannot process another pulse; at high count rates it causes loss of counts and can distort measured activity.

As Low As Reasonably Achievable: the principle that radiation exposure should be minimized by time, distance, and shielding while still accomplishing the clinical objective.

The thickness of a material that reduces the intensity of a photon beam to half its original value; used to compare beam quality and to plan shielding.

A device containing molybdenum-99 bound to a column from which technetium-99m is eluted with saline; the parent's 66-hour half-life allows repeated milking for about two weeks.

The presence of molybdenum-99 in eluted technetium-99m pertechnetate, measured with a dose calibrator; limits are set because molybdenum-99 has a long half-life and emits high-energy gamma rays.

The fraction of the total radioactivity present in the desired chemical form; impurities such as free pertechnetate or reduced-hydrolyzed technetium degrade image quality and increase absorbed dose.

The radioactivity per unit mass of a radionuclide or labeled compound, often expressed as becquerels per microgram; low specific activity can cause receptor saturation in targeted imaging.

A vial of nonradioactive reagents, typically including stannous ion as a reducing agent, to which pertechnetate is added to form a labeled radiopharmaceutical.

The time required for the amount of radioactivity in a biologic system to fall to half, combining physical decay and biologic clearance; used in dosimetry and imaging schedule planning.

Fluorine-18 labeled deoxyglucose, a glucose analog that is trapped in cells after phosphorylation; it reflects glycolytic activity and is the most common PET tracer.

A beta- and gamma-emitting iodine isotope used for imaging and therapy of thyroid disease; its beta emission delivers local tissue dose while the gamma emission permits scanning.

A gamma camera in which a large scintillation crystal is viewed by an array of photomultiplier tubes whose signals are weighted to compute the position of each event.

A method that compensates for photon loss within the patient so that measured counts better reflect true activity distribution, most often using a transmission or CT map.

An analytic reconstruction method that projects counts back along acquisition lines after applying a filter to reduce blurring; fast but sensitive to noise and artifacts.

An iterative reconstruction algorithm that divides data into subsets and updates the image estimate repeatedly; it models counting statistics and is widely used in SPECT and PET.

The axis about which a SPECT camera rotates; misalignment between this axis and the reconstruction center produces ring or arc artifacts and requires calibration.

Acquisition synchronized to the electrocardiogram so that data are binned into cardiac phases; used to assess left ventricular function and perfusion simultaneously.

The uniformity of detector response across the field of view measured with a uniform source; nonuniformity causes ring artifacts in reconstructed SPECT images and requires correction.

The process in which a positron combines with an electron, converting their mass into two 511-keV photons emitted in nearly opposite directions; the basis of PET imaging.

Electronic acceptance of two detector events occurring within a short timing window, assumed to arise from the same annihilation; reduces random and scattered counts.

Measurement of the small arrival-time difference between annihilation photons to localize the event along the line of response, improving signal-to-noise ratio in large patients.

Standardized uptake value: lesion activity concentration normalized to injected dose and patient size, providing a semiquantitative index that is influenced by many technical factors.

Underestimation of activity in structures smaller than about two to three times the spatial resolution, caused by blurring of counts beyond object boundaries.

Imaging with ligands targeting prostate-specific membrane antigen, commonly labeled with gallium-68 or fluorine-18, used for staging and restaging prostate cancer.

Use of a low-dose CT acquisition to create an attenuation map for correcting PET emission data; CT artifacts and contrast can introduce errors in the corrected image.

The paired use of a diagnostic radiotracer and a structurally related therapeutic radionuclide to select patients and deliver targeted treatment, as with gallium-68 and lutetium-177 dotatate.

A formalism for calculating absorbed dose to target organs from source region time-activity data, combining decay scheme, geometry, and residence time information.

Peptide receptor radionuclide therapy: systemic delivery of a radiolabeled somatostatin analog, usually lutetium-177 dotatate, to treat well-differentiated neuroendocrine tumors.

A radionuclide that decays by emitting alpha particles, which have short range and high linear energy transfer, producing dense local DNA damage with limited bystander dose.

Reduced thyroidal iodine uptake after a diagnostic radioiodine dose, which can impair subsequent therapy; a reason to minimize diagnostic iodine-131 activity.

Imaging of the secondary x-rays produced when beta particles are slowed in tissue, used to verify distribution of pure beta emitters such as yttrium-90.

Bone marrow suppression causing cytopenias after radionuclide therapy; blood counts are monitored and doses adjusted according to marrow reserve and prior treatments.