Which statement best describes how the inverse square law applies when the source-to-image distance is halved?

• A. Exposure remains the same
• B. Exposure doubles
• C. Exposure is reduced by one-quarter
• D. Exposure increases by a factor of four

Answer: (D)

Exposure changes with distance according to the inverse square law: intensity is inversely proportional to the square of the distance from the source. Halving the distance means replacing d with d/2, so the exposure becomes proportional to 1/(d/2)^2 = 4/d^2, which is four times the original exposure. In other words, bringing the source closer concentrates the x-ray beam and increases receptor exposure by a factor of four. This is why, in practice, when SID is reduced, you must adjust mAs downward to keep the image receptor exposure within the desired range. The other statements don’t fit because exposure would not stay the same, nor simply double, nor decrease by a quarter under halving the distance.