Which factor primarily controls the number of x-ray photons produced?

• A. Kilovoltage peak (kVp)
• B. Exposure time (s)
• C. Milliamperage (mA)
• D. Focal spot size

Answer: (C)

The number of x-ray photons produced is determined by how many electrons are driven toward the anode each second. That rate is set by the tube current, measured in milliamperes. Increasing the milliamperage increases the electron current and thus the photons generated per unit time, raising the overall photon count. During an exposure, the total photons are the product of this rate and the exposure duration, often expressed as milliampere-seconds. So milliamperage is the primary lever for photon quantity, while exposure time scales the total yield over the exposure. Kilovoltage peak affects photon energy (quality), not how many photons are produced, and focal spot size mainly influences heat distribution and image sharpness rather than photon quantity.