Which factor most directly determines receptor exposure on the radiograph?

• A. Kilovoltage peak
• B. Source-to-image distance
• C. Grid ratio
• D. Milliampere-seconds

Answer: (D)

Receptor exposure is driven by how many x-ray photons actually reach the image receptor. The primary control of that photon quantity is the milliampere-seconds, which sets the photon flux produced by the tube during the exposure. Increasing mAs increases the number of photons and thus increases receptor exposure linearly (doubling mAs doubles receptor exposure; halving mAs halves it).

Kilovoltage peak mainly changes the beam’s energy and patient penetration, influencing image contrast and scatter, but does not directly set the photon quantity reaching the receptor. Source-to-image distance changes the irradiated beam at the receptor via the inverse-square law—longer distance reduces exposure, but this is a geometric effect and is typically balanced by adjusting mAs. Grid ratio affects image quality by reducing scatter and attenuates some primary photons, so it can necessitate an mAs change to maintain exposure, but it’s not the primary driver of receptor exposure itself.

So the key factor that most directly determines receptor exposure is the milliampere-seconds.