Final answer:
To produce a similar radiograph at 48" instead of 60", the mAs required is calculated using the inverse square law, resulting in 12.8 mAs.
Step-by-step explanation:
To calculate the mAs required for a similar radiograph at a different SID (Source-to-Image Distance), we can use the inverse square law. This law suggests that intensity is inversely proportional to the square of the distance. Therefore, if we increase the distance, the intensity decreases, and if we decrease the distance, the intensity increases.
In this case, the distance is changing from 60" to 48". Since 20 mAs produced a satisfactory radiograph at 60", we need to find the correct mAs for 48" while keeping the same radiographic density. Using the inverse square law, the formula to adjust mAs when changing distance is:
mAs1 / (SID1)^2 = mAs2 / (SID2)^2
We can solve for mAs2 (the mAs needed at 48"):
- mAs1 = 20 mAs (at 60")
- SID1 = 60 inches
- SID2 = 48 inches
Solving the equation:
mAs2 = (mAs1 ∙ (SID2)^2) / (SID1)^2
mAs2 = (20 mAs ∙ (48)^2) / (60)^2
mAs2 = (20 mAs ∙ 2304) / 3600
mAs2 = 1280 / 3600
mAs2 = 12.8 mAs
Therefore, 12.8 mAs is needed to create a similar radiograph at 48" SID.