General Nuclear Medicine (CNMT)
Core concepts
The general CNMT domain tests the full arc of a study: radiopharmacy, instrumentation, patient preparation, imaging technique, and interpretation of common planar and SPECT exams. You must be fluent in decay modes, half-life math, radiopharmaceutical biodistribution, and the collimator/energy-window choices that make an image diagnostic.
Know the workhorses: 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, Tc-99m MAA for lung perfusion, and I-123 or Tc-99m for thyroid uptake. Pair each agent with its clearance pathway and normal biodistribution.
Instrumentation and QC
Understand photopeak and energy window settings, uniformity floods, resolution and linearity, and why a low-energy high-resolution collimator suits Tc-99m while medium-energy suits In-111 and I-131. Know daily vs. weekly QC expectations and the purpose of each test.
Traps
Questions love to invert relationships: higher collimator resolution costs sensitivity; a wider window raises counts but degrades contrast; a longer acquisition improves statistics but risks motion. Watch for pediatric dose adjustments, breastfeeding precautions with MAA and iodine agents, and blocking agents (SSKI, perchlorate, Lugol's) used to protect the thyroid.
Rule of thumb
Decay math: activity halves each half-life. For Tc-99m (6 hr), 24 hours is four half-lives, so 800 MBq becomes 50 MBq. For I-131 (8 days), 24 days is three half-lives, so 370 MBq becomes about 46 MBq.