What is the effect of increasing kVp on subject contrast?

• A. Increasing kVp increases subject contrast.
• B. Increasing kVp only affects image brightness.
• C. Increasing kVp reduces subject contrast.
• D. Increasing kVp has no effect on subject contrast.

Answer: (C)

Increasing kilovoltage raises the energy of the X-ray photons, making the beam more penetrating. As energy goes up, the interaction that dominates shifts from photoelectric effects (which vary a lot with tissue type and create strong differences in attenuation) toward Compton scattering (which is less dependent on tissue composition). That shift reduces the differences in attenuation between different tissues, so the subject contrast on the image decreases. You may see a brighter overall image if exposure isn’t adjusted, but the visible contrast between adjacent tissues is reduced.