Hybrid Imaging: PET/CT & PET/MRI – Vocabulary Flashcards

Vocabulary flashcards covering key terms and concepts from Professor Anderson’s lecture on PET basics and hybrid imaging.

Positron Emission Tomography (PET)

Nuclear imaging technique that detects paired 511 keV photons from positron–electron annihilation to create cross-sectional images of radiotracer distribution.

Annihilation Coincidence Detection (ACD)

PET method that records an event only when two opposing detectors simultaneously measure the back-to-back annihilation photons, enabling electronic collimation.

Back-to-back annihilation photons

The two 511 keV gamma rays emitted 180° apart when a positron annihilates with an electron.

511 keV photon

Single gamma ray produced during positron annihilation; two are detected in coincidence in PET imaging.

Positron-emitting radionuclide

Radioisotope that decays by emitting a positron, making it suitable for PET (e.g., F-18, C-11).

Carbon-11

Cyclotron-produced PET isotope; half-life 20.4 min; max positron energy 960 keV; positron fraction 0.99.

Nitrogen-13

Cyclotron isotope; half-life 9.96 min; max positron energy 1.19 MeV; positron fraction 1.00.

Oxygen-15

Cyclotron isotope; half-life 123 s; max positron energy 1.72 MeV; positron fraction 1.00.

Fluorine-18

Cyclotron isotope; half-life 110 min; max positron energy 635 keV; high clinical use; positron fraction 0.97.

Gallium-68

Generator-derived isotope (from Ge-68); half-life 68.2 min; max positron energy 1.9 MeV; positron fraction 0.88.

Rubidium-82

Generator-derived isotope (from Sr-82); half-life 78 s; max positron energy 3.35 MeV; positron fraction 0.95.

Half-life

Time required for half the atoms of a radionuclide sample to decay.

Maximum positron energy

Highest kinetic energy carried by an emitted positron; influences positron range and PET image resolution.

Pixel

Smallest 2-D picture element in a digital image matrix; size equals DFOV divided by matrix dimension.

Voxel

Three-dimensional volume element defined by pixel size and slice (or detector) thickness.

Display Field of View (DFOV)

Physical extent of anatomy displayed in the reconstructed image; used to calculate pixel size.

Matrix (imaging)

Grid of rows and columns (e.g., 512 × 512) into which the image is digitized.

Coincidence Logic

Electronics that compare time stamps from opposing detectors to decide whether two events form a valid coincidence.

Time Stamp (PET)

Digital record of the exact moment a detector registers a photon, usually with 1–2 ns precision.

Coincidence Timing Window

Preset interval (≈6–12 ns) within which two recorded events must fall to be accepted as a coincidence.

Timing Resolution

Ability of a PET system to measure time differences between photon detections; typically 0.5–5 ns.

Electronic Collimation

Localization of events by coincidence timing rather than physical lead collimators, giving PET high sensitivity.

Detector Ring

Stationary 360° array of scintillator detectors surrounding the patient, allowing simultaneous acquisition from all angles.

Time-of-Flight (TOF) PET

Technique estimating annihilation position along a line of response by measuring arrival-time difference of the two photons.

Depth of Interaction (DOI) effect

Resolution degradation caused by uncertainty in where within a thick scintillator crystal the photon was absorbed.

Effective Positron Range

Average perpendicular distance between the decay site and annihilation point of the positron.

Non-colinearity

Small angular deviation (~±0.25°) from 180° between annihilation photons due to residual momentum at annihilation.

Spatial Resolution

Smallest distance at which two objects can be distinguished; limited in PET by positron range, non-colinearity, detector size, etc.

Full Width at Half Maximum (FWHM)

Width of a system’s response profile at 50 % of its peak; common measure of spatial resolution.

Sampling (PET)

Recording of projection data; ideally ≥3 samples per FWHM, though 2 per detector element are common clinically.

Reconstruction Filter Cut-off Frequency

Highest spatial frequency retained during image reconstruction; lower count studies use lower cut-offs, reducing resolution.

PET Sensitivity

Ability of a scanner to detect annihilation events; depends on detector absorption efficiency and solid-angle coverage.

Geometric Efficiency

Fraction of emitted photon pairs that intersect a detector pair; maximal at the center of the line of response.

Multicoincidence Operation

Mode in which each detector element forms coincidences with many opposite elements, increasing sensitivity and uniformity.

Block Detector

PET detector module where a scintillator block is optically divided into small elements read by four photomultiplier tubes.

Quadrant Sharing

Block-detector modification using larger PMTs that overlap adjacent blocks, reducing the total number of PMTs needed.

Phoswitch Detector

Block design with stacked layers of two scintillators having different decay times to identify depth of interaction.

Bismuth Germanate (BGO)

Dense scintillator with ~95 % detection efficiency for 511 keV photons; widely used in PET.

Lutetium Oxyorthosilicate (LSO)

Fast, dense scintillator (40 ns decay) with ~88 % efficiency for 511 keV photons.

Lutetium–Yttrium Oxyorthosilicate (LYSO)

Y-doped version of LSO offering similar efficiency (~88 %) and higher light output.

Gadolinium Oxyorthosilicate (GSO)

Scintillator with ~70 % efficiency and 60 ns decay; often paired with LSO in phoswitch designs.

Sodium Iodide (NaI(Tl))

Traditional gamma-camera scintillator; only ~34 % efficient for 511 keV photons, limiting its PET application.

Barium Fluoride (BaF2)

Fast scintillator with ~44 % efficiency for 511 keV photons; investigated for TOF PET.

Annihilation Photon

One of the pair of 511 keV gamma rays produced when a positron and electron annihilate.

Generator-produced radionuclide

PET isotope obtained from decay of a long-lived parent in an in-house generator, e.g., Ga-68 or Rb-82.