NMTCB(RS) cheat sheet

Tc-99m has a 6.02-hour physical half-life and 140.5 keV gamma emission; the Mo-99/Tc-99m generator is eluted with 0.9% saline, and Mo-99 breakthrough is limited to 0.15 kBq Mo-99 per MBq Tc-99m (0.15 µCi/mCi).

Effective half-life: 1/Teff = 1/Tphysical + 1/Tbiological; the radionuclide decays to 50% activity in one physical half-life (A = A0 × e^(−0.693t/T½)).

Standard adult Tc-99m MDP bone scan dose is about 740–925 MBq (20–25 mCi) IV, with imaging delayed ~2–4 hours; hydration and post-injection voiding reduce bladder and soft-tissue activity.

Ventilation/perfusion lung scan: perfusion uses Tc-99m MAA (particle size 10–90 µm, ~200,000–500,000 particles); a high-probability PE pattern is ≥2 large segmental mismatched defects with normal ventilation.

Meckel diverticulum imaging uses Tc-99m pertechnetate; sensitivity is improved by pentagastrin or H2 blockers, and false positives include ectopic gastric mucosa, intussusception, and urinary activity.

Hepatobiliary (HIDA) scan uses Tc-99m mebrofenin/IDA agents; a positive result for acute cholecystitis is nonvisualization of the gallbladder with normal hepatic uptake and biliary transit by 60 minutes.

GI bleeding localization uses Tc-99m sulfur colloid (rapid hepatic clearance, detects ~0.1 mL/min) or Tc-99m labeled RBCs (longer imaging window, detects ~0.04–0.1 mL/min).

Thyroid uptake uses I-123 (123–370 MBq oral) with 24-hour normal uptake about 10–30%; Graves disease shows elevated uptake, subacute thyroiditis shows low uptake.

Radiation dosimetry: effective dose is the sum of tissue equivalent doses weighted by tissue weighting factors; ALARA limits are occupational 50 mSv/year (5 rem), public 1 mSv/year.

CT numbers (Hounsfield units): HU = 1000 × (µtissue − µwater)/µwater; water = 0, air ≈ −1000, dense bone ≈ +1000 or higher.

Window width controls contrast (narrow width = higher contrast); window level sets the center gray value; bone windows use wide width (~2000) and high level (~400–500).

Pitch = table travel per gantry rotation / total collimated beam width; pitch >1 reduces dose and scan time but can degrade image quality if too high.

CT dose index: CTDIvol reflects dose per unit volume (mGy); dose-length product DLP = CTDIvol × scan length (mGy·cm); effective dose ≈ DLP × k (body ~0.015 mSv/mGy·cm).

Automatic exposure control (tube current modulation) varies mA by patient attenuation; it reduces dose but must be paired with appropriate reference mAs and noise index.

Reconstruction: filtered back projection is fast but noisier; iterative reconstruction reduces noise and dose but can alter texture and low-contrast detectability.

Beam hardening and metal artifacts are reduced by higher kVp, thinner slices, and metal artifact reduction algorithms; photon starvation appears as streaks in high-attenuation regions.

Multi-detector CT: thinner detector rows improve z-axis resolution; reconstruction slice thickness can differ from acquisition thickness; overlapping reconstruction improves MPR quality.

Contrast timing: bolus tracking or test bolus determines arterial/venous phases; renal function (eGFR) and prior contrast reaction history guide iodinated contrast decisions.