Photon Interactions with Matter

Photoelectric, Compton, and coherent — see which interaction dominates at each energy and what that means for dose and contrast.

30 keV in soft tissue — Compton predominates

Above 20 keV, Compton scattering is the more likely interaction in soft tissue. Compton does not care about atomic number, so it carries no contrast information — it just fogs the image and puts scatter in the room. Photoelectric probability has fallen away as 1/E³ while Compton has only fallen as 1/E, which is why raising kVp trades contrast for penetration.

Bushong puts the crossover — where the two are equally probable — at 20 keV in soft tissue.

Coherent scatteringalso called classical or Thompson scattering

Happens below about 10 keV — under the diagnostic range
The x-ray changes direction but not wavelength, so not energy
No energy is transferred, so no ionization occurs
Most coherently scattered x-rays travel on forwards

Photoelectric effecttotal absorption of the incident photon

The incident x-ray is completely absorbed — nothing carries on
It cannot happen unless the x-ray has at least the electron’s binding energy
Probability rises with the cube of atomic number (Z³) — this is what makes contrast
Probability falls with the cube of energy (1/E³), so it drops away fast as kVp rises

Compton scatteringthe interaction that produces scatterPredominates here

The x-ray is deflected and loses energy, and the atom is ionized
Independent of atomic number — as likely in bone as in soft tissue
Probability falls with energy (1/E), far more slowly than photoelectric
Even at 180 degrees the scattered x-ray keeps about two-thirds of its energy

Source: Bushong, Radiologic Science for Technologists, 12e, Ch. 10

Safety lessonInteractions with matter

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