CAMRT Radiography Practice Exam

If the exposure used at 30 SID is 5 mAs, what would the mAs exposure be at 40 SID?

1. 5 mAs

2. 6 mAs

3. 8.88 mAs

4. 10 mAs

The correct answer is: 8.88 mAs

To understand the change in mAs required when adjusting the source-to-image distance (SID), it is important to recognize the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. This means that as the distance from the x-ray tube increases, the intensity of the x-ray beam decreases. In this situation, the original exposure is at 30 inches SID with an mAs of 5. When increasing the distance to 40 inches, the mAs must be adjusted to maintain the same image receptor exposure. To calculate the new mAs at the increased SID, you can use the following formula derived from the inverse square law: New mAs = Old mAs x (New SID^2 / Old SID^2) Substituting the provided values into this formula: New mAs = 5 mAs x (40^2 / 30^2) This simplifies to: New mAs = 5 mAs x (1600 / 900) = 5 mAs x (16/9) = 5 mAs x 1.777... ≈ 8.88 mAs Thus, the correct answer is approximately