Why does filtration improve image contrast?

• A. By removing low-energy photons that contribute to patient dose without adding useful signal, thereby reducing scattered and soft radiation.
• B. By increasing the number of high-energy photons, increasing brightness.
• C. By focusing beams to a smaller spot.
• D. By reducing focal spot heat.

Answer: (A)

Filtration improves image contrast by removing the low-energy, or “soft,” photons from the x-ray beam. Those soft photons are easily absorbed in superficial tissues and contribute to the patient dose without providing useful information to the image. They also increase scatter and fog within the patient, which drags down contrast at the detector. By filtering them out, the beam becomes harder (higher average energy), reducing unnecessary dose and the amount of soft radiation and scatter that reach the image receptor. The remaining photons carry more diagnostic signal relative to the background, so differences in attenuation between tissues show up more clearly as better contrast. This effect isn’t achieved by changing the focal spot or increasing beam brightness; filtration specifically trims out those useless low-energy photons.