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 an image has contrast at all, and where the scatter that doses you and fogs the image actually comes from.
Five interactions between x-rays and matter are described in physics. At diagnostic energies only two of them happen often enough to matter: one absorbs the x-ray completely, and one deflects it in a new direction carrying less energy. Which of the two dominates depends on the tissue and on the energy of the beam, and the balance between the two sets both the contrast you get and the scatter you have to protect against.
Knowing which interactions cannot happen is worth as many exam points as knowing which do, because the three that never occur in imaging appear constantly as distractors.
| Interaction | Happens in diagnostic imaging? | Why |
|---|---|---|
| Photoelectric effect | Yes | Produces subject contrast |
| Compton scattering | Yes | Produces scatter and fog |
| Coherent (classical) scattering | Barely | A small percentage, no ionization |
| Pair production | No | Needs at least 1.02 MeV |
| Photodisintegration | No | Needs more than about 10 MeV |
A diagnostic beam runs roughly 30 to 150 kVp, so its photons top out near 150 keV. Pair production needs energy nearly seven times that ceiling, and photodisintegration around seventy times it. Neither is possible in a radiographic room, which is exactly why a select-all question about what can occur in the patient uses them.
Answer this to keep going. 9 more sections unlock below.
A 150 kVp chest exposure is made. Which of these interactions cannot occur in the patient?
Section 1 of 10. Answer the check to keep going.