Only about what percent of incident photons emerge from the patient unaffected?

• A. 1%
• B. 5%
• C. 20%
• D. 50%

Answer: (B)

When X-rays pass through a patient, most photons are removed from the beam by absorption or scattering. The photons that come out the other side without interacting are the transmitted, or unaffected, ones. Under typical diagnostic imaging conditions, only about five percent of the incident photons emerge from the patient unchanged. This small transmission fraction is why x-ray images rely on the contrast created by the varying attenuation of tissues. Increasing beam energy (higher kVp) or using thinner/denser areas can raise this transmission, but for a standard exam through a typical patient, the figure is around five percent.