During diagnostic radiography, which interactions contribute to attenuation:

• A. 1 and 2
• B. 1 only
• C. 2 and 3
• D. 1, 2, and 3

Answer: (B)

Attenuation is the reduction in the number of photons traveling along the beam as it passes through matter. In diagnostic radiography, the interaction that most directly removes photons from the forward beam is photoelectric absorption. In this process, the photon is completely absorbed by an atom, ejecting an inner-shell electron and transferring all its energy to the atom; the photon ceases to exist in the forward direction, so the transmitted intensity drops.

Coherent (elastic) scattering and Compton (inelastic) scattering do not remove photons in the same straightforward way from the primary beam path. Coherent scattering deflects photons with little or no energy change and is relatively unlikely at diagnostic energies in soft tissue, contributing mainly to scattered radiation rather than to a loss of the forward beam in the primary path. Compton scattering changes the photon’s direction and energy, but many scattered photons still reach the detector from off-axis paths; this scattered radiation is typically treated as noise or as a separate component of image formation rather than a simple attenuation of the forward beam.

Thus, the interaction that accounts for the primary attenuation of the forward beam in this framing is photoelectric absorption, which is why that option is the best choice.