Nuclear Medicine Glossary
Fifty core terms across physics, instrumentation, radiopharmacy, procedures, quantitation, therapy, safety and reporting, each with a plain definition, why it matters, and an example.
Physics
Becquerel (Bq)
SI unit of activity = 1 nuclear disintegration per second. 1 MBq = 10⁶ Bq; 1 mCi = 37 MBq.
Why: the unit every administered activity is expressed in.
e.g. a bone scan ~600 MBq.
Half-life (T½)
Time for activity to fall by half. Physical (decay), biological (excretion) and effective (combined) versions exist.
Why: sets imaging timing, shelf-life and patient-release timing.
e.g. Tc-99m 6 h, F-18 110 min, I-131 8 d.
Positron (β⁺) annihilation
A positron meets an electron and converts to two 511 keV photons ~180° apart.
Why: the physical basis of PET coincidence detection.
e.g. F-18 FDG.
Isomeric transition
A metastable nucleus drops to a lower energy state emitting a γ with no change in atomic number.
Why: gives Tc-99m its pure 140 keV γ and low dose.
e.g. Tc-99m → Tc-99.
Compton scatter
A photon loses energy deflecting off an electron, arriving mispositioned.
Why: degrades contrast; rejected by the energy window + scatter correction.
Linear energy transfer (LET)
Energy deposited per unit path length. α = high LET (dense, short range); β/γ = low LET.
Why: high-LET α therapy kills efficiently with little marrow toxicity.
e.g. Ra-223 α therapy.
Instrumentation
Collimator
Lead/tungsten septa that only pass near-perpendicular photons, defining resolution and rejecting scatter.
Why: match to isotope energy (LEHR for Tc-99m, high-energy for I-131).
Anger (gamma) camera
NaI(Tl) crystal + PMT array with position/energy logic that forms the planar image.
Why: the core SPECT detector.
Energy window
Range of accepted photon energies around the photopeak (e.g. 140 keV ±10%).
Why: too wide accepts scatter; too narrow loses counts.
SPECT
Single-photon emission CT. Rotating heads reconstruct a 3-D distribution.
Why: localises and quantifies over planar imaging.
Coincidence detection
PET accepts only photon pairs arriving within a few nanoseconds along a line of response.
Why: electronic collimation → higher PET sensitivity/resolution.
Time-of-flight (TOF)
Uses the tiny arrival-time difference to localise the annihilation along the line of response.
Why: improves PET signal-to-noise, especially in large patients.
Dose calibrator
Ionisation chamber measuring activity before administration.
Why: the legal measurement of what the patient receives; needs constancy/linearity/accuracy QC.
Radiopharmacy
Radiopharmaceutical
A radionuclide bound to a carrier molecule that targets a physiological process.
e.g. Tc-99m MDP → bone.
Generator (Mo-99/Tc-99m)
A long-lived parent decays to a short-lived daughter, eluted (“milked”) on demand.
Why: puts Tc-99m in every department without a cyclotron.
Cold kit
Non-radioactive vial of ligand reconstituted with eluate to tag the tracer at point of use.
e.g. MAG3, MDP, MAA kits.
Radiochemical purity (RCP)
Fraction of activity in the desired chemical form, checked by ITLC/chromatography.
Why: free pertechnetate/hydrolysed Tc degrade the image (thyroid/stomach or lung/liver uptake).
Specific activity
Activity per unit mass of the labelled compound.
Why: matters for receptor-targeted agents (avoid saturating the target).
Theranostic pair
Same molecular target imaged with a diagnostic radionuclide then treated with a therapeutic one.
e.g. Ga-68/Lu-177 DOTATATE.
Procedures
Uptake time
Interval from injection to imaging that lets the tracer localise.
e.g. FDG ~60 min; bone ~2–4 h.
Three-phase study
Flow + blood-pool + delayed images, mostly for bone/infection.
Why: separates cellulitis (early) from osteomyelitis (delayed).
Diuretic renography
Furosemide challenge (F+20/F−15/F0) to distinguish obstruction from dilatation.
Why: a curve that washes out with furosemide is not obstructed.
Gated SPECT
Acquisition binned to the ECG to measure wall motion and ejection fraction.
Why: normal motion over a “defect” suggests attenuation, not infarct.
Stress/rest imaging
Paired perfusion sets (pharmacologic or exercise stress + rest).
Why: reversible = ischaemia; fixed = scar.
Sentinel node mapping
Tracer injected around a tumour drains to the first (sentinel) node.
e.g. melanoma, breast Tc-99m nanocolloid.
Quantitation
SUV (standardised uptake value)
ROI activity concentration normalised to injected activity/body weight.
Why: semi-quantifies PET uptake for response assessment.
e.g. SUVmax fall on treatment.
SUL
SUV normalised to lean body mass, less weight-dependent.
Why: used in PERCIST response criteria.
Split (differential) function
Each kidney’s percentage contribution to total function.
e.g. 60:40 on MAG3/DMSA.
Ejection fraction (EF)
Fraction of ventricular volume ejected per beat, from gated SPECT/MUGA.
Why: cardiotoxic-chemo monitoring, viability work-up.
Gallbladder ejection fraction (GBEF)
Percentage GB emptying after CCK on hepatobiliary study.
Why: <35–38% suggests chronic acalculous cholecystitis.
Krenning score
Visual grading of somatostatin-receptor uptake vs liver/spleen.
Why: selects NET patients for Lu-177 DOTATATE.
Therapy
PRRT
Peptide receptor radionuclide therapy: β⁻ emitter on a receptor-targeting peptide.
e.g. Lu-177 DOTATATE for NET.
Radioembolisation (SIRT)
Y-90 microspheres delivered intra-arterially to liver tumours.
Why: needs Tc-99m MAA work-up for lung shunt + non-target flow.
Remnant ablation
I-131 to destroy residual thyroid tissue after thyroidectomy for DTC.
e.g. ~1.1–3.7 GBq.
Dosimetry
Calculating absorbed dose to target and organs at risk to individualise therapy.
Why: protects kidneys/marrow in PRRT and PSMA therapy.
Written directive
Physician’s signed order specifying radionuclide, activity and route for therapy (US NRC).
Why: a legal prerequisite before administration.
Post-therapy scan
Imaging that uses the therapy activity’s γ to map distribution.
e.g. I-131 whole-body at 2–7 d.
Safety
ALARA / ALARP
Keep doses As Low As Reasonably Achievable/Practicable.
Why: the optimisation principle behind every protocol.
Diagnostic reference level (DRL)
An audit benchmark activity for a standard study, not a dose limit.
Why: consistently exceeding it triggers optimisation.
Effective dose
Tissue-weighted whole-body dose in mSv, for comparing risk across procedures.
e.g. FDG PET ~7 mSv (+ CT).
Patient release
Rules allowing a radioactive patient to go home (US: <5 mSv to others; written instructions).
e.g. I-131 ~1221 MBq/33 mCi releasable.
Thyroid block
Potassium iodide/perchlorate to stop the thyroid taking up free radioiodine.
Why: mandatory for I-123/I-131 agents (DaTscan, mIBG).
Comforter & carer dose constraint
A dose ceiling (often ~5 mSv/episode) for people knowingly helping a radioactive patient.
Why: distinct from public/worker limits.
Reporting
PERCIST
PET Response Criteria in Solid Tumours: SUL-based treatment response.
Why: standardises “better/worse” on FDG-PET.
Deauville score
5-point lymphoma PET score referencing mediastinum + liver uptake.
e.g. score 4–5 = above-liver residual.
PSMA-RADS / PROMISE
Structured frameworks for reporting PSMA-PET findings + likelihood.
Why: consistent staging + therapy selection.
Superscan
Diffuse intense skeletal uptake with faint/absent kidneys on bone scan.
e.g. widespread prostate mets or metabolic bone disease.
Mismatch / match
V/Q: perfusion defect with normal ventilation (mismatch) suggests PE; cardiac PET: flow–metabolism mismatch = viable hibernating myocardium.
Incidentaloma
Unexpected focal uptake needing correlation (physiological vs pathological).
Why: FDG-avid colonic/thyroid foci may need endoscopy/biopsy.
High Yield Imaging · Glossary. Educational reference. Definitions are concise teaching summaries; consult SNMMI/EANM guidance and standard texts for full detail.
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Educational reference for imaging professionals. It does not replace local policy, manufacturer labelling or clinical judgement.