TEMN Medicina Nuclear cheat sheet

Radiopharmaceutical = radionuclide (source of radiation) + carrier molecule (determines biodistribution); ideal agent has high target-to-background ratio, short effective half-life, and decays to a stable daughter.

Effective half-life: 1/Teff = 1/Tfísico + 1/Tbiológico; Teff is always shorter than either component alone and governs dose delivered per unit activity.

Technetium-99m: T½ físico ≈ 6 h, emisión gamma de 140 keV, obtenido del generador 99Mo/99mTc; el 99Mo tiene T½ ≈ 66 h y la elución debe respetar el rendimiento del generador.

Radionuclide purity = fraction of activity from the intended radionuclide; radiochemical purity = fraction of activity in the intended chemical form; both must be verified before administration.

Fluor-18 (FDG): T½ ≈ 110 min, emisión de positrones; la captación refleja transporte de glucosa (GLUT) y hexoquinasa, no simplemente metabolismo glucolítico.

FDG normal uptake: brain cortex, myocardium (variable, depends on fasting state), liver (reference), spleen, bone marrow, kidneys, bladder, tonsils, salivary glands, bowel; fasting ≥4–6 h suppresses myocardial and insulin-driven uptake.

Renal clearance dominates excretion of most 99mTc agents; hepatobiliary clearance applies to iminodiacetic acid derivatives (99mTc-IDA) and to 99mTc-sulfur colloid (RES uptake).

Dose optimization: ALARA plus diagnostic reference levels (DRLs); pediatric activity scaled by weight or EANM pediatric dose card, not simply halved adult doses.

Pre-examination checklist: pregnancy/lactation status, renal function (eGFR), recent iodinated contrast or barium, thyroid-blocking medication, and prior nuclear studies that could cause carry-over.

Gamma camera components: collimator → NaI(Tl) crystal → photomultiplier tubes → preamplifier/amplifier → pulse-height analyzer → computer; the collimator defines spatial resolution and sensitivity.

Collimator choice: low-energy high-resolution (LEHR) for 99mTc imaging; medium-energy for 111In/67Ga; high-energy for 131I; pinhole for small organs (thyroid) with magnification.

Energy resolution of NaI(Tl) ≈ 6–7% at 140 keV; a 15–20% window centered on the photopeak is standard for 99mTc, wider for higher-energy emitters.

Spatial resolution worsens with increasing source-to-collimator distance; sensitivity improves with larger collimator holes but resolution degrades - an inverse trade-off.

Uniformity and linearity must be checked with a flood phantom; intrinsic uniformity is measured without the collimator, extrinsic (system) uniformity with it.

SPECT reconstruction: filtered back-projection is fast but noisy; iterative ordered-subset expectation maximization (OSEM) with attenuation and scatter correction is preferred for quantification.

PET detection relies on annihilation photons (511 keV) in coincidence within a timing window; time-of-flight and point-spread-function modeling improve lesion detectability.

Attenuation correction in SPECT uses CT (SPECT/CT) or transmission sources; without it, deep structures such as the inferior myocardial wall and liver dome appear falsely decreased.

Dead-time and count-rate losses occur at high activity; keep syringe residual activity low and image after adequate decay to avoid saturation artifacts.

Myocardial perfusion: 99mTc-sestamibi/tetrofosmin or 201Tl; reversible defect = ischemia, fixed defect = scar/infarct; gated SPECT adds ejection fraction and wall motion.

Ventilation/perfusion (V/Q) scan: mismatched segmental or subsegmental perfusion defect with normal ventilation indicates pulmonary embolism; matched defects suggest parenchymal disease.

Bone scan (99mTc-MDP): uptake proportional to osteoblastic activity and blood flow; three-phase technique separates blood flow, blood pool, and delayed bone phases; superscan shows diffuse high uptake with faint renal visualization.

Thyroid: 99mTc-pertechnetate or 123I uptake; increased uptake with diffuse enlargement suggests Graves disease, focal hot nodule suggests autonomous adenoma, cold nodule requires cytologic evaluation.

Renal scintigraphy: 99mTc-DTPA for GFR, 99mTc-MAG3 for effective renal plasma flow, 99mTc-DMSA for cortical morphology; captopril renography assesses renovascular hypertension.

Sentinel lymph node mapping uses 99mTc-labeled colloid with intraoperative gamma probe; lymphoscintigraphy maps drainage basins in melanoma and breast cancer.

PET/CT with FDG: staging and restaging of lymphoma, lung cancer, melanoma, head-and-neck tumors, and unknown primary; SUVmax is semiquantitative and depends on lesion size, uptake time, and blood glucose.

False-positive FDG uptake: infection, inflammation, granulomatous disease, brown fat, muscle activity, and recent surgery/radiotherapy; false-negative: small lesions, mucinous tumors, hyperglycemia.

Therapy with unsealed sources: 131I for hyperthyroidism and differentiated thyroid carcinoma, 177Lu-DOTATATE for neuroendocrine tumors, 223Ra for bone metastases in prostate cancer, 90Y microspheres for hepatic tumors.

Dose quantities: atividade em becquerel (1 Bq = 1 desintegração/s; 1 mCi = 37 MBq); dose absorvida em gray; dose equivalente e efetiva em sievert, com fatores de ponderação por tipo de radiação e tecido.

Radiation weighting: photons and electrons wR = 1; alpha particles wR = 20; neutrons vary with energy; equivalent dose = absorbed dose × wR.

Time–distance–shielding: exposure rate follows the inverse-square law; doubling distance quarters the exposure; lead or tungsten shielding is used for high-energy emitters and PET.

Occupational dose limits: 20 mSv/year averaged over 5 years (100 mSv per 5 years) with no single year exceeding 50 mSv; public limit 1 mSv/year; extremity limit 500 mSv/year.

Pregnancy: the embryo/fetus dose limit is 1 mSv for occupational exposure; before any administration, exclude pregnancy in women of reproductive age and discuss breastfeeding interruption per radionuclide.

Contamination control: gloves, absorbent bench covers, fume hood for volatile agents (131I), survey meters for gamma emitters and wipe tests for removable contamination.

Waste decay: store short-lived waste (99mTc, 18F) until activity falls below clearance levels (commonly 10 half-lives); long-lived waste follows licensed disposal.

Quality control of the gamma camera: daily uniformity and energy peaking, weekly resolution/linearity, quarterly or annual full calibration; dose calibrator constancy, accuracy, and linearity checks.

Justification and optimization: every study must have a clinical question; use the lowest activity consistent with diagnostic image quality and avoid repeat imaging without new indication.

Stochastic effects (cancer, heritable disease) have no threshold and probability increases with dose; deterministic effects (skin burns, cataract, marrow suppression) have thresholds and severity increases with dose.

Typical thresholds for deterministic effects: transient erythema ~2 Gy, cataract ~0.5 Gy (lens), hematopoietic syndrome ~1–2 Gy whole-body, gastrointestinal syndrome ~5–10 Gy.

Linear no-threshold model is used for radiation protection; effective dose from a typical bone scan is a few mSv, comparable to a few years of background radiation.

Acute radiation syndrome: prodromal symptoms, then latent period; management includes supportive care, colony-stimulating factors, and consultation with radiation medicine specialists.

Radionuclide contamination: remove contaminated clothing, wash skin with soap and water, monitor with survey meter, and collect urine/feces for excretion measurement; internal contamination may require blocking agents (stable iodine for 131I, Prussian blue for cesium).

Extravasation of radiotracer: stop injection, apply cold compress, elevate the limb, document activity and time, and estimate the local dose for follow-up.

Anaphylactoid reactions to radiopharmaceuticals are rare; treat with airway support, oxygen, fluids, and antihistamines per institutional protocol.

Pregnancy and lactation counseling: estimate fetal dose per administered activity, consider alternative imaging, and provide specific breastfeeding interruption times (e.g., 99mTc agents usually a few hours; 131I requires cessation).

Record keeping: administered activity, lot number, radiochemical purity results, adverse events, and dose estimates must be documented for traceability and audit.