General Nuclear Medicine (CNMT): Core Concepts, Radiopharmacy, and Exam Traps
What the CNMT domain tests
General nuclear medicine questions blend instrumentation, radiopharmacy, radiation safety, and organ-specific imaging. You must be fluent in the physics of detection (photoelectric absorption, Compton scatter, and how pulse-height analyzers window the photopeak), collimator selection (parallel-hole low-energy high-resolution vs. high-sensitivity; pinhole for small structures; converging for magnification), and camera quality control (daily uniformity/flood, energy-peak calibration, resolution and linearity checks).
Radiopharmacy essentials
Know the common kits and their normal biodistribution: Tc-99m MDP for bone, Tc-99m sulfur colloid for liver/spleen and marrow, Tc-99m mebrofenin for hepatobiliary, Tc-99m DTPA or MAG3 for renal, Tc-99m pertechnetate for thyroid and Meckel studies, and I-123 or Tc-99m for thyroid uptake. Understand generator elution (Mo-99 breakthrough testing, Al-99m breakthrough testing), radionuclidic vs. radiochemical purity, and the 12-hour eluate expiry concept. Know that free pertechnetate localizes to thyroid, stomach, and salivary glands, which explains many artifacts.
Traps that appear in questions
Distractors often swap the energy window (Tc-99m 140 keV, I-123 159 keV, Ga-67 multiple peaks, F-18 511 keV), confuse decay schemes (Tc-99m is a metastable isomer; I-131 emits beta), or misstate patient prep (nothing by mouth before gastric emptying; thyroid blocking before I-131; hydration and voiding before bone scan). Watch for half-life questions: Tc-99m 6 hours, F-18 110 minutes, I-123 13 hours, I-131 8 days, Tl-201 73 hours.
Rule of thumb
If a question asks why an image looks wrong, check the chain in order: patient prep and injection quality, radiochemical purity, collimator/energy window match, then camera QC. Most artifacts trace to the first two links.