Approximately 5% of the incident x-ray photons that emerge from the patient to form the radiographic image are known as what?

• A. Remnant radiation
• B. Secondary beam
• C. Stray radiation
• D. Primary beam

Answer: (A)

Remnant radiation is the portion of the primary X-ray beam that actually exits the patient and contributes to forming the radiographic image. After the beam passes through the body, most photons are attenuated, so only a small fraction—about 5%—emerges toward the image receptor. These exiting photons may be unaltered but transmitted primary photons or photons produced by scattering within the patient. This is different from the primary beam (the original photons leaving the tube before interacting with the patient), the secondary beam (photons generated by interactions within matter that contribute to scatter but aren’t the direct image-forming remnant), and stray radiation (unwanted radiation outside the useful beam). Hence, the 5% remnant radiation is the photons that form the image.