NMTCB PET cheat sheet

Annihilation of a positron with an electron produces two 511 keV photons emitted ~180° apart; the small angular deviation (<1°) plus positron range blurs spatial resolution.

Detector block = scintillation crystal array coupled to photomultiplier tubes; Anger logic computes the interaction position from PMT signal ratios.

Time-of-flight PET requires fast scintillators (e.g., LSO, LYSO) with coincidence timing resolution in the hundreds of picoseconds, improving SNR rather than spatial resolution.

Coincidence timing window is typically 4–12 ns; a narrower window rejects random coincidences but risks losing true events from detector timing jitter.

Random coincidence rate ≈ 2·τ·R1·R2 (τ = timing window); randoms rise with the square of activity, so they dominate at high count rates.

Sensitivity is the fraction of emitted annihilation pairs detected; it depends on solid angle, detector thickness, and energy window, not on injected dose.

NECR = T²/(T + S + 2R) quantifies noise-equivalent count rate; it peaks at moderate activity and falls as randoms and dead time grow.

Scatter correction methods include energy-window-based (dual/triple energy windows), model-based convolution-subtraction, and Monte Carlo simulation.

Attenuation correction uses transmission sources (e.g., rotating 68Ge line source) or CT-based mu-maps; CT mu-maps must be converted from CT numbers to 511 keV attenuation coefficients.

18F-FDG is taken up by glucose transporters and trapped after hexokinase phosphorylation (FDG-6-phosphate); it is not further metabolized in most tissues.

18F has a 109.8-minute half-life and is produced in a cyclotron via the 18O(p,n)18F reaction on enriched 18O-water.

68Ga has a 67.7-minute half-life and is eluted from a 68Ge/68Ga generator; 68Ge (271 d) breaks through gradually and must be tested.

68Ga-DOTATATE binds somatostatin receptors (SSTR2) and is used for neuroendocrine tumors; physiologic uptake occurs in spleen, liver, pituitary, and adrenals.

18F-NaF localizes to bone remodeling sites via fluoride exchange with hydroxylapatite; it is used for bone metastases and is cleared renally.

11C has a 20.4-minute half-life and requires on-site cyclotron; 15O has a 2.0-minute half-life and is used for perfusion studies.

Radiochemical purity is verified by chromatography (e.g., ITLC or HPLC); 18F-FDG USP requires ≥90% radiochemical purity and pH 4.5–7.5.

FDG uptake is suppressed by hyperglycemia and insulin; patients fast ≥4–6 hours and glucose should be <200 mg/dL before injection.

Standard FDG oncology dose is 10–20 mCi (370–740 MBq) IV; uptake period is 60 minutes (range 45–75), and imaging follows bladder emptying.

Patient preparation: fast ≥4–6 hours, avoid strenuous exercise 24 hours before, and withhold caffeine if protocol requires; document glucose level.

FDG normal biodistribution: brain, myocardium (variable), liver, spleen, kidneys, ureters, bladder, and GI tract; brown fat in supraclavicular/paravertebral regions.

CT parameters for attenuation correction use low-dose settings (e.g., 120 kVp, 30–80 mAs); diagnostic CT may be added for lesion characterization.

SUV = tissue activity concentration / (injected dose / body weight); lean-body-mass or BSA normalization reduces variability in obese patients.

A negative FDG PET does not exclude malignancy; false negatives occur with small lesions (<1 cm), low metabolic rate tumors, and hyperglycemia.

Inflammatory/infectious uptake, granulomas, and postsurgical changes can mimic malignancy; correlate with history and prior imaging.

Myocardial FDG imaging for viability requires dietary preparation (high-fat, low-carbohydrate) plus prolonged fasting to shift metabolism to fatty acids.

Daily QC: detector uniformity (blank scan or uniform phantom), energy window peak position, and coincidence timing check before patient imaging.

Weekly/monthly QC: detector gain and timing drift, crystal energy resolution, and coincidence timing resolution verification per manufacturer schedule.

Normalization scan corrects for detector efficiency variations; it must be acquired after detector changes or significant repairs.

Well counter QC: daily background and constancy checks with a long-lived source (e.g., 137Cs or 68Ge); chi-square test verifies counting statistics.

Dose calibrator QC: daily constancy with a reference source, annual accuracy and linearity checks, and geometry/volume correction.

Uniformity phantom scan should show <5% variation across the FOV; ring artifacts suggest a defective detector or normalization error.

Spatial resolution is measured with a point source or line source; FWHM in reconstructed images is typically 4–6 mm for modern scanners.

PET/CT alignment QC uses a point source at multiple bed positions to verify CT and PET coordinate registration within 1–2 mm.

Occupational dose limits: 50 mSv/year (5 rem) effective dose; lens 150 mSv/year; extremities 500 mSv/year.

Public dose limit is 1 mSv/year (100 mrem); ALARA principles require time, distance, and shielding optimization.

Inverse-square law: exposure decreases with the square of distance; doubling distance reduces dose to one-quarter.

Time, distance, and shielding: lead aprons attenuate 511 keV poorly (~10–20% with 0.5 mm Pb), so distance is the primary protection.

Pregnant workers may declare pregnancy and are limited to 0.5 mSv (500 mrem) to the fetus over the gestation period.

Radioactive package receipt requires survey of external dose rate, contamination wipe test, and documentation of package labeling.

Patient release criteria: dose rate at 1 meter typically <7 mrem/h; written instructions must be provided for breastfeeding and close contact.

Medical event (misadministration) reporting: notify NRC within 24 hours if dose differs by ≥20% from prescribed or if wrong radiopharmaceutical is administered.

PET radiopharmaceuticals are regulated under 10 CFR Part 35; written directives and authorized user supervision are required for administration.