NMTCB PET key terms

The process in which a positron emitted by a radionuclide collides with an electron, converting both masses into two 511 keV photons that travel in nearly opposite directions.

A positively charged antimatter electron emitted during beta-plus decay. It travels a short distance in tissue before annihilating with an electron, producing the paired photons PET detects.

The 511 thousand electron-volt gamma ray created when a positron and electron annihilate. Its energy equals the rest mass of an electron, and PET detectors are tuned to this energy.

The straight path connecting two detectors that record coincident 511 keV photons. The annihilation event is assumed to lie somewhere along this line during image reconstruction.

Electronic logic that accepts two detector signals as a true event only when they arrive within a preset timing window, typically a few nanoseconds, indicating a single annihilation.

The short time interval, usually 4 to 12 nanoseconds depending on system design, within which two detector pulses must occur to be recorded as a coincidence event.

An accidental event formed when photons from two separate annihilations are detected within the timing window. Randoms raise background and are reduced by shorter windows and dead-time correction.

A coincidence in which one or both photons Compton-scatter in the patient before detection, changing direction and depositing some energy. Scatter blurs contrast and is limited by energy windows.

Interaction in which a photon transfers part of its energy to an electron and deflects at an angle. In PET it degrades localization and lowers the photon's detected energy.

Interaction in which a photon transfers all its energy to an electron and disappears. In PET detectors this full-energy deposit is the ideal event used for energy discrimination.

A selected range of pulse heights, commonly around 350 to 650 keV, accepted as valid 511 keV events. It rejects low-energy scatter and very high-energy noise.

A vacuum device that converts scintillation light into an electrical signal and amplifies it. PMTs are coupled to scintillator crystals in many PET scanners.

A solid-state light sensor made of many tiny avalanche photodiode cells. SiPMs replace PMTs in newer PET systems, offering compact size and operation in strong magnetic fields.

A material such as LSO, LYSO, BGO, or GSO that emits visible light when a gamma ray deposits energy. Crystal properties affect sensitivity, timing, and energy resolution.

A PET capability that measures small arrival-time differences between the two coincidence photons to estimate where along the line of response the annihilation occurred, improving image contrast.

The uncertainty, expressed in picoseconds, with which a scanner measures the arrival-time difference of coincidence photons. Better timing narrows TOF localization along the line of response.

The distance a positron travels in tissue before annihilating. Longer ranges, as with rubidium-82, blur spatial resolution because the annihilation is displaced from the decay site.

The smallest distance between two points that a scanner can display as separate. In PET it is limited by detector size, positron range, and non-collinearity of annihilation photons.

Correction for photon loss as 511 keV rays pass through tissue. It uses a transmission or CT-derived map and is essential for accurate quantification in PET images.

Conversion of a CT image, acquired at lower x-ray energies, into a 511 keV attenuation map. It speeds whole-body PET but can introduce artifacts from contrast or metal.

A hybrid scanner combining positron emission tomography with computed tomography. The CT provides anatomic detail and attenuation correction for the functional PET data.

A positron-emitting radionuclide with a 110-minute half-life, commonly produced in a cyclotron and used to label FDG and other tracers for PET imaging.

A glucose analog labeled with fluorine-18 that enters cells and is trapped after phosphorylation. Its uptake reflects glucose metabolism and is used widely in oncology and inflammation imaging.

A particle accelerator that bombards a target with protons to produce positron-emitting radionuclides such as fluorine-18, carbon-11, and nitrogen-13 for radiopharmaceutical synthesis.

A self-contained system that produces a short-lived radionuclide from a longer-lived parent. Examples include rubidium-82 from strontium-82 and gallium-68 from germanium-68.

A potassium analog with a 76-second half-life, eluted from a strontium-82 generator, used for myocardial perfusion PET. Its short half-life enables rapid rest-stress protocols.

A positron emitter with a 68-minute half-life, obtained from a germanium-68 generator or cyclotron. It labels peptides such as DOTATATE for neuroendocrine tumor imaging.

A somatostatin receptor-binding peptide labeled with gallium-68 or copper-64. It localizes neuroendocrine tumors that overexpress somatostatin receptors.

A positron-emitting perfusion tracer with a 10-minute half-life used for myocardial blood flow imaging. Its short half-life requires an on-site cyclotron or nearby supplier.

A PET study using rubidium-82 or ammonia N-13 to assess regional blood flow to the heart muscle, often combined with stress testing to detect ischemia.

The waiting time after injection before scanning, allowing the radiopharmaceutical to distribute and localize. For FDG it is typically about 60 minutes with quiet rest.

Instructions before a PET scan, such as fasting, avoiding vigorous exercise, and controlling blood glucose for FDG. Proper preparation reduces confounding uptake and repeat scans.

Physiologic FDG accumulation in brown adipose tissue, often in the neck and supraclavicular region, triggered by cold or stress. It can mimic malignancy and is reduced by warming and relaxation.

A semiquantitative measure of tracer concentration normalized to injected dose and body size. It helps compare lesion uptake over time and across studies.

Underestimation of activity in objects smaller than the scanner resolution, because counts spread beyond the true lesion. It lowers measured SUV in small lesions.

A tumor arising from hormone-producing cells that often overexpresses somatostatin receptors. It is imaged with gallium-68 DOTATATE PET to guide staging and therapy.

Tests performed at the start of each imaging day, such as detector uniformity and energy peak checks, to confirm the scanner is stable before patient studies begin.

A calibration that corrects for differences in detector efficiency across the scanner so that a uniform source produces a uniform image. It is repeated periodically.

A metric combining true, random, and scatter count rates to express image quality per unit imaging time. It guides selection of the optimal activity and acquisition settings.

The principle of keeping radiation exposure as low as reasonably achievable, considering social and economic factors. It drives shielding, time, distance, and dose-reduction practices.

Calculation of radiation dose absorbed by patients or staff from administered activity and exposure conditions. It supports safe dosing and compliance with regulatory limits.

Material such as lead or concrete used to reduce radiation exposure. In PET facilities, shielding design accounts for the high penetration of 511 keV photons.