Approximately what percentage of the incident photons will emerge from the patient unaffected to form the radiographic image?

• A. 5%
• B. 20%
• C. 80%
• D. 95%

Answer: (A)

This question relies on understanding attenuation of the x-ray beam as it passes through the patient. Most photons interact within body tissues—through absorption or scattering—so only a small fraction come out the other side unaltered to form the radiographic image.

Typically, only about five percent of the incident photons emerge intact. This low transmission is why images rely on the remaining photons that are absorbed or scattered inside the body to create contrast. If energy were much higher, more photons would pass through without interacting, but even then the fraction that emerges unaltered remains a minority; hence five percent is a commonly cited approximate value for transmission.

Twenty percent would imply substantially more photons make it through without interaction than is typical, while eighty or ninety-five percent would suggest most photons pass through unchanged, which would reduce image contrast and is not how attenuation generally behaves in clinical imaging.