In the patient’s exit path of photons, what percent of incident photons emerge from the patient unaffected?

• A. 5%
• B. 2%
• C. 10%
• D. 20%

Answer: (A)

Photons that pass straight through tissue without interacting are those that are transmitted. In diagnostic radiography, most photons are absorbed or scattered as they traverse the body, so only a small fraction emerge unaltered along the exit path. The transmitted portion is described by T = e^(-μx), where μ is the tissue’s attenuation coefficient and x is thickness. For typical patient thickness and energy ranges (80–120 kVp), this transmission is on the order of a few percent, commonly around five percent. The remaining photons are either absorbed or scattered, which is why the exit dose is much lower than the incident dose.