Diagnostic Radiological Physics
Core physics for diagnostic imaging
A candidate must be fluent in x-ray production (bremsstrahlung and characteristic spectra, the role of kVp in shaping the spectrum and mAs in scaling output), beam attenuation (linear and mass attenuation coefficients, half-value layer, the exponential attenuation law), and the interaction mechanisms that dominate in the diagnostic range: photoelectric absorption and Compton scatter. Understand how atomic number and photon energy shift the relative probability of each, because this drives contrast and dose tradeoffs.
Image formation and quality
Know the components of image quality-contrast, spatial resolution, noise, and their coupling to dose. Be able to relate mAs to quantum noise (noise scales inversely with the square root of photons), kVp to contrast and penetration, and focal spot size and geometry to blur. Understand detector types (indirect conversion with scintillator plus photodiode versus direct conversion), fill factor, and detective quantum efficiency as the summary metric of dose efficiency.
Traps to watch
Questions often test whether you know that increasing kVp lowers subject contrast but reduces dose, or that the photoelectric effect dominates at low energy and high Z. Another common trap is confusing HVL with penetration quality versus quantity of photons. Watch for items that ask about scatter fraction rising with patient thickness and field size.
Worked example / rule of thumb
Attenuation: if HVL = 0.5 cm, after 2 cm the transmitted fraction is (1/2)^(2/0.5) = (1/2)^4 = 6.25%. Rule of thumb: each HVL removes half the photons; count HVLs, then halve repeatedly.
Study guidance
Practice converting between linear and mass attenuation coefficients (divide by density) and predicting how beam hardening shifts the spectrum toward higher effective energy as it passes through tissue.