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Exam category: Safety → Radiation Physics and Radiobiology
Why it matters: These are the interactions that happen inside the patient, and they explain two separate things at once — why images have contrast, and where scatter radiation (staff dose) actually comes from.
Once the beam enters the patient, only two interactions really matter in diagnostic imaging: one absorbs the x-ray completely, the other scatters it in a new direction with less energy. Which one dominates depends heavily on the tissue and the beam energy — and that balance is the entire reason contrast and scatter behave the way they do.
An incident x-ray ejects an inner-shell electron (the photoelectron) and is completely absorbed — nothing scatters onward. This process is strongly dependent on atomic number: higher-Z materials (bone, iodine/barium contrast) absorb far more via photoelectric effect than soft tissue does. This Z-dependence is the reason bone and contrast media show up so clearly against soft tissue — it's the main source of subject contrast.
A photoelectric interaction also produces a low-energy characteristic x-ray as a byproduct (an outer-shell electron drops into the vacancy) — but this secondary x-ray is far too low-energy to reach the image receptor or exit the patient.
An incident x-ray ejects an outer-shell electron (the Compton electron, ionizing the atom) and continues on in a new direction with reduced energy — this scattered x-ray is the actual source of occupational scatter dose and image fog. Unlike the photoelectric effect, Compton scattering probability is roughly the same across soft tissue and bone — it depends on electron/mass density much more than atomic number.
A Compton-scattered x-ray can deflect anywhere up to 180° (backscatter), and even at full 180° deflection it retains roughly two-thirds of its original energy — meaning scattered x-rays can still carry meaningful dose and still fog the image.
Only occurs at very low energies (below ~10 keV) — the incident x-ray excites the atom, which releases a photon of the same energy in a new direction, no ionization involved. Of little practical importance in the diagnostic energy range.
The overall reduction in beam intensity as it passes through tissue depends on two things: the thickness of the body part and the type of tissue (atomic number/density). This is why a thicker or denser body part attenuates more of the beam — and why technique has to be adjusted accordingly.
Students assume higher atomic number always means "more scatter." It's the opposite logic that matters: atomic number drives photoelectric absorption (contrast), not Compton scattering, which is comparatively Z-independent. Don't let a question's mention of "bone vs. soft tissue" pull you toward a Compton-based answer when it's really testing photoelectric/contrast.
Source: Bushong, Radiologic Science for Technologists, 12th ed., Ch. 10.
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