Physics of Diagnostic Radiology
Core concepts
Understand the imaging chain as a sequence: x-ray production, beam filtration, patient interaction, scatter rejection, detection, and display. Key physics includes the bremsstrahlung vs characteristic spectrum, the effect of kVp on beam quality and quantity, mAs and the inverse-square law, and the difference between exposure, absorbed dose, and effective dose. For CT, know Hounsfield unit definition, pitch, reconstruction kernels, and how mA modulation and iterative reconstruction affect noise and dose. For MRI, focus on T1/T2 relaxation, TR/TE weighting, gradient vs spin echo, and safety zones. Ultrasound candidates must know axial vs lateral resolution, the effect of frequency on penetration, and Doppler angle dependence.
Traps
Questions love to conflate quantities: doubling kVp does not double dose, and increasing mAs raises dose linearly without changing beam energy. Remember that filtration hardens the beam (raises effective energy, lowers patient skin dose for the same output). In CT, pitch greater than 1 lowers dose but can increase noise; thinner slices increase noise. In MRI, a short TR with short TE gives T1 weighting, not T2. Doppler shift depends on the cosine of the insonation angle, so a 90-degree angle yields no detectable shift.
Worked rule of thumb
For a fixed receptor dose, use the 15% rule: a 15% increase in kVp roughly doubles receptor exposure, so mAs can be halved to maintain image brightness. Conversely, dropping kVp by 15% requires about double the mAs. This is a frequent numeric distractor: the correct answer preserves exposure while acknowledging that contrast increases at lower kVp.