If a radiograph is taken at a 40 cm focal-film distance, what mAs is required if the distance is increased to 80 cm while maintaining the same kVp?

To understand the correct answer regarding the required mAs when changing the focal-film distance from 40 cm to 80 cm while keeping the kVp constant, we can apply the inverse square law. This principle states that the intensity (or exposure) of radiation decreases with the square of the distance from the source.

When the distance is doubled, as in this case from 40 cm to 80 cm, the exposure needed to maintain the same level of radiation (for the same image quality) increases quadratically. Specifically, if the distance is doubled, the required mAs must increase by a factor of four because:

1. The exposure at a given distance is proportional to the square of the distance.
2. Therefore, if the distance is increased from 40 cm to 80 cm, the factor would be (80 cm / 40 cm)² = 2² = 4.

This means that you would need to quadruple the original mAs to get the same exposure at the greater distance.

Assuming the original mAs at 40 cm was set to a base value of 2, the increased mAs required to maintain the same exposure at 80 cm would be 2 (original mAs) × 4