Post-Primary Computed Tomography (NMTCB(CT)): Physics, Protocols, and Hybrid Imaging
What the CT domain tests
Post-primary CT questions assume you already know nuclear medicine and test CT physics, radiation dose, contrast, and image quality. Core physics includes the x-ray tube (tungsten anode, heel effect), filtration, collimation, detector arrays, and the reconstruction pipeline (filtered back projection vs. iterative reconstruction). You must understand CT numbers (Hounsfield units: water 0, air about -1000, bone hundreds to >1000), window width and level, and how pitch, slice thickness, and kVp/mA interact with dose and noise.
Dose and image quality
Know the dose metrics: CTDIvol (per-slice dose index), DLP (dose-length product), and effective dose (DLP times a conversion factor). Understand ALARA and automatic exposure control. Recognize trade-offs: lower pitch raises dose; thinner slices raise noise and dose; higher kVp lowers contrast but reduces noise and photon starvation artifacts. Identify common artifacts: beam hardening (dark streaks between dense objects), photon starvation (shoulders), motion, metal/streak, and partial volume.
Contrast and hybrid workflow
Distinguish oral vs. IV contrast, iodinated contrast risks (allergy, nephropathy), and the role of non-contrast CT for attenuation correction in PET/CT and SPECT/CT. Know that CT attenuation correction improves localization and quantification but can introduce artifacts from dense contrast or metal. Understand CT protocol selection: pediatric dose reduction, breath-hold instructions, and coverage matched to the emission scan.
Trap to watch
Questions love to swap CTDIvol and DLP or ask which parameter changes dose without changing noise. Remember: mAs controls noise and dose linearly; kVp affects both contrast and dose non-linearly (roughly kVp squared to the power of ~2.5).
Rule of thumb
When asked how to reduce dose without hurting diagnostic quality, lower mAs with iterative reconstruction and automatic exposure control before touching kVp.