CAMRT Radiography Practice Exam

If the distance changes from 60" to 72", what adjustment in mAs is needed according to the square law?

1. Decreased by about 10%

2. Increased by 5%

3. Increased to approximately 9.1 mAs

4. Decreased to approximately 6.3 mAs

The correct answer is: Increased to approximately 9.1 mAs

The square law for radiation states that the intensity of radiation is inversely proportional to the square of the distance from the source. As the distance increases, the intensity of the radiation decreases, and consequently, to maintain the same exposure, the mAs (milliamperes seconds) must be adjusted. When the distance changes from 60 inches to 72 inches, you can use the formula: \[ mAs_{1} \cdot \frac{d_{1}^2}{d_{2}^2} = mAs_{2} \] Here, let's denote the original mAs at 60 inches as mAs1. The distance d1 is 60 inches, and d2 is 72 inches. You would plug in the numbers: \[ mAs_{2} = mAs_{1} \cdot \frac{(60)^2}{(72)^2} \] Calculating the square of the distances: \[ \frac{(60)^2}{(72)^2} = \frac{3600}{5184} \approx 0.6944 \] This indicates that you need to multiply the original mAs by approximately 0.694