If a radiograph is initially created with 125 milliampere-seconds (mAs) at a 40-inch source-to-image distance, what new mAs should be used if the exam is repeated at a 36-inch SID to maintain exposure?

To determine the appropriate new milliampere-seconds (mAs) when changing the source-to-image distance (SID) while maintaining the same exposure, the inverse square law is applied. This law states that the intensity of radiation is inversely proportional to the square of the distance from the source.

In this scenario, the initial SID is 40 inches, and the new SID is 36 inches. To find the new mAs needed to maintain exposure, you can use the formula:

New mAs = Old mAs × (Old SID² / New SID²)

This means you can plug in the values:

Old mAs = 125 Old SID = 40 inches New SID = 36 inches

Now substituting into the formula:

New mAs = 125 × (40² / 36²) = 125 × (1600 / 1296) = 125 × 1.2345679 (approximately) = 154.3 (which can be rounded to 154)

It appears that the calculated new mAs, about 154, matches the reasoning for needing to increase the mAs with a decrease in SID to maintain exposure.

However, if the answer chosen is 101 m