Which type of photons are preferentially removed by filtration?

• A. High energy photons
• B. Low energy photons
• C. Medium energy photons
• D. All photons equally

Answer: (B)

Filtration is designed to trim off photons that contribute little to image formation but add dose—primarily the soft, low-energy photons. The filter material is more effective at absorbing these low-energy photons because the interaction probability (the attenuation cross-section) is higher at low energies, largely due to photoelectric absorption dominating in that range. High-energy photons pass through the filter more readily, so they’re not removed as much. The net effect is a harder beam with fewer low-energy photons. That's why low-energy photons are preferentially removed.