If an x-ray was taken at a 36-inch SID using a mAs of 12, what new mAs would result in the same image receptor exposure at a 48-inch SID?

To determine the mAs that would result in the same image receptor exposure at a different source-to-image distance (SID), it is necessary to understand the inverse square law as it applies to x-ray exposure. The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from the source.

When the SID increases, the radiation intensity delivered to the image receptor decreases. Specifically, to maintain the same exposure at a longer distance, the mAs must be adjusted accordingly. The formula used to relate the different mAs values at different SIDs is:

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

In this scenario, the old SID is 36 inches, and the new SID is 48 inches. The initial mAs is 12. Plugging in the values, we can calculate:

New mAs = 12 mAs × (36^2 / 48^2)

Calculating this step by step:

1. Calculate the squares of the SIDs:

• 36^2 = 1296

• 48^2 = 2304

2. Substitute these values into the formula:

• New mAs =