A radiograph taken with 4 mAs at a SID of 37 inches needs to be repeated at a 45-inch SID. What new mAs should be used to maintain the same image receptor exposure?

To maintain the same image receptor exposure when changing the source-to-image distance (SID), you must adjust the milliampere-seconds (mAs) according to the inverse square law. The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from the source.

When increasing the SID from 37 inches to 45 inches, you will need to increase the mAs to compensate for the increased distance, so the receptor receives the same amount of exposure.

The formula used to calculate the new mAs is as follows:

New mAs = Original mAs × (New SID)^2 / (Old SID)^2

Substituting the values:

New mAs = 4 mAs × (45 inches)^2 / (37 inches)^2

= 4 mAs × (2025 / 1369)

≈ 4 mAs × 1.477 = 5.908 mAs

Rounding this value, the closest answer to maintain sufficient exposure is 6 mAs. Thus, this adjustment ensures the radiograph is of comparable exposure to the original despite the changes in distance.