Which of the following changes will not reduce the receptor exposure by one-half when starting with 24 mAs?

Reducing receptor exposure by one-half typically involves altering the mAs (milliampere-seconds) or modifying factors such as kVp or exposure time. When starting with 24 mAs, to achieve a reduction to half, you would need to set the exposure to 12 mAs.

Choosing 18 mAs reduces the original exposure, but not by half; instead, it represents a decrease to 75% of the original exposure (18 mAs is greater than 12 mAs). This would not achieve the requirement for halving the receptor exposure.

In contrast, adjusting the exposure time to 1/50 second or changing the kVp to 72 would be viable options, as they can result in an actual reduction in receptor exposure by the desired amount. Likewise, increasing the mA to 300 could also lead to changes in exposure, depending on the duration of the exposure, but it would not intrinsically reduce receptor exposure either.

Therefore, the choice of 18 mAs does not fulfill the condition of reducing the receptor exposure by one-half when starting at 24 mAs, making it the correct answer in this context.