What new mAs should be used to maintain image receptor exposure when changing from a 40-inch to a 36-inch SID?

To maintain image receptor exposure when changing the source-to-image distance (SID) from 40 inches to 36 inches, it is important to understand the relationship between distance and exposure. The exposure to the image receptor is influenced by the inverse square law, which states that as the distance from the radiation source increases, the intensity of the radiation decreases proportionately to the square of the distance.

Here, you are decreasing the SID from 40 inches to 36 inches. This reduction in distance will increase the intensity of the radiation reaching the image receptor. To compensate for this change and maintain consistent exposure, the mAs (milliampere-seconds) must be adjusted accordingly.

When using the inverse square law for these distances, the equation looks like this:

New mAs = Old mAs x (Old SID / New SID)².

If the original mAs is assumed as 125 mAs at a 40-inch SID, then substituting the values in will show that the new mAs to maintain similar exposure at 36 inches results in approximately 101 mAs. Thus, this adjustment accounts for the increased radiation intensity due to the shorter distance.

The reasoning behind arriving at 101 mAs focuses on maintaining consistent image quality while