Which statement identifies primary radiation?

• A. Radiation produced within the patient
• B. Radiation scatter from interactions within the patient
• C. Radiation that escapes the protective housing
• D. Radiation that exits the window in the tube housing

Answer: (D)

Primary radiation is the useful x-ray beam created at the focal spot and exiting through the tube window. These photons travel toward the patient and the image receptor before any interactions occur outside the tube, forming the beam that produces the radiographic image. Radiation produced inside the patient and scatter within the patient are secondary radiation, generated after the primary photons interact with tissue. Leakage radiation would be photons escaping through the protective housing rather than through the window. Because the defining feature of primary radiation is that it exits the tube window as the directed, useful beam, the statement describing radiation leaving the window identifies primary radiation.