What primary exposure control determines the amount of radiation reaching the image receptor?

• A. kVp
• B. mAs (milliampere-seconds)
• C. Focal spot size
• D. Exposure time only

Answer: (B)

The amount of radiation that reaches the image receptor is determined by how many photons are produced in the X-ray tube. That photon quantity is controlled by the tube current and how long the exposure lasts, combined as mAs. So, the primary exposure control is mAs—the product of mA (tube current) and exposure time. Doubling mAs doubles the number of photons reaching the receptor, increasing receptor exposure; halving mAs halves it.

kVp changes the energy of the photons (their penetrating power) and also influences contrast, but it is not the primary factor that sets how much radiation reaches the receptor. Focal spot size affects image sharpness and heat capacity, not the overall photon quantity reaching the receptor. The option of “exposure time only” is incomplete because receptor exposure depends on the combination of current and time (mAs), not time alone.