Physics, Instrumentation, and Radiologic Technology
Core concepts
You must be fluent in X-ray production (tube voltage, mA, exposure time, filtration, focal spot size and the line-focus principle), beam quality versus quantity, and how kVp shifts the spectrum. Understand attenuation as the sum of photoelectric absorption and Compton scatter, and how each varies with photon energy and atomic number - this underpins contrast. Know detector technology: CR photostimulable phosphor plates versus indirect (scintillator + photodiode) and direct (photoconductor) flat-panel detectors, and the meaning of DQE, MTF, and detective quantum efficiency trade-offs. For CT, understand Hounsfield unit definition, pitch, collimation, reconstruction kernels, filtered back projection versus iterative reconstruction, and the relationship between slice thickness, noise, and dose. For MRI, know proton precession, Larmor frequency, T1/T2 relaxation, pulse sequences (spin echo, gradient echo, EPI), and the origin of artifacts. For ultrasound, know piezoelectric transduction, frequency-resolution-penetration trade-offs, and Doppler principles.
Traps
Questions love the inverse relationships: higher kVp lowers contrast but reduces patient dose for a given receptor exposure; larger focal spot improves heat capacity but degrades spatial resolution; thinner CT slices increase noise unless mA is raised. Distinguish spatial resolution (line pairs/mm) from contrast resolution. Remember that increasing mAs raises quantity, not quality. In MRI, a short TE with long TR gives proton density weighting, not T1.
Rule of thumb
At the 15% kVp rule: a 15% increase in kVp approximately doubles receptor exposure (halves required mAs) but reduces subject contrast. Use it to reason about dose-contrast trade-offs in exam vignettes.