Radiologic Physics
This domain tests the physics behind the image: how x-rays are produced, how they interact with matter, and the electrical principles behind the generator. Items often ask you to classify an interaction, convert energy and wavelength, or predict how a change in kVp alters the spectrum.
Production and the spectrum
- Bremsstrahlung produces the continuous spectrum and most of the photons; characteristic emission produces discrete peaks at the tungsten K-shell binding energies (about 59-69 keV).
- kVp sets the maximum photon energy and shapes the spectrum; mA and exposure time set the number of photons; filtration shifts the spectrum toward higher average energy (beam hardening).
- Energy relations: E = hf = hc/wavelength; 1 eV = 1.602 x 10 to the -19 J; shortcut E(keV) = 12.4 / wavelength in angstroms.
Interactions with matter
- Photoelectric absorption: complete absorption, dominant at low energy and high atomic number; raises subject contrast and patient dose.
- Compton scatter: dominant in soft tissue at diagnostic energies; depends on electron density rather than atomic number; degrades contrast and raises occupational dose.
- Coherent (Rayleigh) scattering: no ionization.
- Pair production requires more than 1.02 MeV and does not occur at diagnostic energies.
Circuits and attenuation
- Ohm law V = IR; power P = VI; series and parallel rules apply to circuit questions.
- Step-up transformer for the tube circuit, step-down for the filament; rectification converts alternating current to direct current, and ripple affects output.
- Attenuation: I = I0 x e to the -mu x; half-value layer is the thickness that halves intensity.
Worked example
A photon has a wavelength of 0.25 angstrom. E = 12.4 / 0.25 = 49.6 keV. Reasoning: the shortcut derives from E = hc/wavelength and lets you convert without SI units, which is exactly the speed the exam expects.
Traps
- Saying kVp changes the quantity of photons rather than their energy distribution.
- Mixing up photoelectric and Compton contributions to contrast and dose.
- Forgetting that added filtration increases HVL, reduces intensity, and lowers patient dose.
- Applying pair production to a 90 kVp beam.