If an x-ray tube delivers 3 milliroentgen per 1 mAs at 40 inches of distance, what will be the radiation output at 80 inches of distance?

The rationale for identifying 1 mR/mAs as the correct answer lies in the principle of the inverse square law, which governs radiation intensity relative to distance. According to this law, the intensity of radiation is inversely proportional to the square of the distance from the source of radiation.

Initially, at a distance of 40 inches, the x-ray tube produces 3 milliroentgen per 1 mAs. If we double the distance to 80 inches, we apply the inverse square law to determine how the intensity changes.

At 40 inches, the intensity is 3 mR/mAs. When the distance is increased to 80 inches, the new intensity can be calculated as follows:

• The distance is increased from 40 inches to 80 inches, which is a factor of 2.

• According to the inverse square law, when the distance doubles, the intensity becomes one-fourth of the original intensity.

Therefore, the calculation shows:

3 mR/mAs ÷ (2^2) = 3 mR/mAs ÷ 4 = 0.75 mR/mAs.

The closest option to 0.75 mR/mAs, rounding to the nearest whole number, is 1