NMTCB PET flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Annihilation radiation energy

Positron-electron annihilation produces two 511 keV photons emitted approximately 180 degrees apart. The 511 keV value equals the electron rest mass energy (0.511 MeV), which is the basis for PET detection.

Coincidence detection principle

Two detectors registering photons within the coincidence timing window (typically 4-12 ns) are assumed to originate from one annihilation event along the line of response between them.

Time-of-flight PET

Fast scintillators measure the arrival-time difference of the two 511 keV photons, localizing the annihilation along the line of response and improving signal-to-noise ratio, especially in large patients.

NECR formula

Noise-equivalent count rate NECR = T squared divided by (T + S + 2R), where T is trues, S is scatter, and R is randoms. It peaks at moderate activity and declines at high count rates.

Random coincidence rate

Random rate is approximately 2 times the timing window times the product of the singles rates in the two detectors. It increases with the square of activity and is reduced by a narrower timing window.

Attenuation correction sources

Transmission-based correction uses a rotating germanium-68 line source; CT-based correction converts CT Hounsfield units to 511 keV attenuation coefficients via a bilinear or piecewise conversion.

PET spatial resolution

Resolution is limited by positron range, photon non-collinearity (about 0.5 degrees), detector crystal size, and reconstruction. Modern scanners achieve roughly 4-6 mm FWHM.

Scintillator crystals used in PET

Common crystals include LSO, LYSO, GSO, and BGO. LSO and LYSO offer high light output and fast decay for time-of-flight; BGO has higher stopping power but slower timing.

18F-FDG mechanism

FDG enters cells via glucose transporters and is phosphorylated by hexokinase to FDG-6-phosphate, which is trapped because it is not a substrate for further glycolysis or glycogen synthesis.

18F half-life and production

Fluorine-18 has a 109.8-minute half-life and is produced in a cyclotron by the oxygen-18 (p,n) fluorine-18 reaction using enriched oxygen-18 water.

68Ga-DOTATATE target

This tracer binds somatostatin receptor subtype 2 and is used for neuroendocrine tumors. Normal uptake appears in spleen, liver, pituitary, adrenals, and kidneys.

18F-NaF mechanism

Fluoride exchanges with hydroxyl groups on bone hydroxyapatite, concentrating at sites of bone remodeling. It is used for bone metastases and clears through the kidneys.

Radiochemical purity testing

Purity is measured by chromatography such as ITLC or HPLC to confirm the fraction of radioactivity in the desired chemical form. USP requires at least 90 percent for FDG.

Generator breakthrough testing

For a germanium-68/gallium-68 generator, eluate is tested for germanium-68 breakthrough, which increases with generator age. Limits are set by USP and must be documented before use.

Hyperglycemia effect on FDG

Elevated blood glucose competes with FDG for transporters and insulin shifts tracer to muscle, reducing tumor uptake. Fasting at least 4-6 hours and glucose below 200 mg/dL are standard.

Carbon-11 and oxygen-15

Carbon-11 has a 20.4-minute half-life and oxygen-15 has a 2.0-minute half-life. Both require an on-site cyclotron because of rapid decay.

Standard FDG dose and uptake

Adults typically receive 10-20 mCi (370-740 MBq) intravenously, followed by a 60-minute uptake period in a quiet, warm room before imaging.

FDG patient preparation

Patients fast 4-6 hours, avoid strenuous exercise for 24 hours, and have blood glucose measured and documented. Warmth and quiet reduce brown fat and muscle uptake.

Normal FDG biodistribution

Physiologic uptake includes brain, myocardium (variable), liver, spleen, kidneys, urinary tract, and bowel. Brown fat appears in supraclavicular and paravertebral regions.

SUV calculation

Standardized uptake value equals tissue activity concentration divided by injected dose decay-corrected to scan time divided by body weight. Lean body mass or BSA normalization reduces variability.

FDG false negatives

Small lesions under 1 cm, low-grade or low-metabolic tumors, hyperglycemia, and recent high-dose steroids can suppress uptake and produce false-negative FDG PET studies.

Myocardial viability preparation

Viability imaging uses a high-fat, low-carbohydrate diet and prolonged fasting to shift myocardial metabolism to fatty acids, so ischemic but viable myocardium retains FDG uptake.

CT attenuation correction settings

Low-dose CT for attenuation correction typically uses about 120 kVp and 30-80 mAs. A separate diagnostic CT may be acquired for anatomic detail.

Daily PET quality control

Daily checks include detector uniformity, energy window peak position, and coincidence timing verification. Results are compared with baseline values before patient imaging begins.

Normalization scan purpose

A normalization scan corrects differences in detector efficiency across the ring. It must be repeated after detector replacement or major repair to prevent ring artifacts.

Dose calibrator constancy test

A long-lived reference source such as cesium-137 or germanium-68 is measured daily to verify calibrator response. Annual accuracy, linearity, and geometry tests are also required.

Uniformity phantom criteria

A uniform cylinder phantom scan should show variation under about 5 percent across the field of view. Ring artifacts indicate detector or normalization problems.

PET/CT image registration QC

A point source imaged at several bed positions verifies that PET and CT coordinates align, typically within 1-2 mm, preventing misregistration of attenuation correction.

Occupational dose limits

Annual limits are 50 mSv effective dose, 150 mSv to the lens, and 500 mSv to extremities. ALARA requires minimizing time, maximizing distance, and using shielding.

Inverse-square law

Radiation exposure decreases with the square of distance from a point source. Doubling distance reduces exposure to one-quarter, making distance the most effective protection at 511 keV.

Pregnant worker fetal limit

A declared pregnant worker is limited to 0.5 mSv (500 mrem) to the fetus over the entire gestation period, and the declaration must be voluntary and in writing.

Medical event reporting

A medical event is reported to the NRC within 24 hours when the administered dose differs by 20 percent or more from the prescribed dose or the wrong radiopharmaceutical is given.