Which interaction contributes the most scattered photons that reach the image receptor?

• A. Photoelectric absorption
• B. Compton scatter
• C. Coherent scatter
• D. Pair production

Answer: (B)

In diagnostic X-ray energies, the photons that become scatter reaching the image receptor mainly come from Compton scatter. This interaction—the photon hits a loosely bound outer electron, is deflected, and retains enough energy to travel toward the detector—has a higher probability in tissue at typical clinical energies than the other processes. Photoelectric absorption removes photons from the beam instead of scattering them toward the receptor, so it contributes to dose and image contrast through absorption differences rather than to scatter reaching the image receptor. Coherent (Rayleigh) scatter also occurs but with a smaller probability and tends to produce less radiation reaching the detector. Pair production requires energies above about 1 MeV, well beyond the diagnostic range, so it doesn’t contribute to scatter at all.