CAMRT Radiography Practice Exam

If you are performing a mobile chest x-ray at a distance of 72" using an mAs of 9.1, what was the original mAs used at a distance of 60"?

1. 4.0 mAs

2. 6.3 mAs

3. 8.0 mAs

4. 10.5 mAs

The correct answer is: 6.3 mAs

To determine the original mAs used at a distance of 60 inches when the mAs at 72 inches is 9.1, we can use the inverse square law of radiation, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. This principle allows us to calculate the adjusted mAs based on the change in distance. When calculating mAs using the distance, the formula used is: \[ mAs_1/mAs_2 = (D_2/D_1)^2 \] where \( D_1 \) and \( D_2 \) are the original and new distances respectively, and \( mAs_1 \) and \( mAs_2 \) are the respective mAs values. For this example: - \( D_1 = 72 \) inches (the distance at which the current mAs is given) - \( D_2 = 60 \) inches (the distance at which we need to find the original mAs) - \( mAs_2 = 9.1 \) Substituting the values into the formula gives: \[ mAs_1/9.1 = (72/60)^2