Final answer:
To produce a similar radiograph when reducing the SID from 60" to 48", the required mAs decreases to 12.8 mAs, calculated using the direct square law.
Step-by-step explanation:
The student's question is related to the principles of radiographic imaging, specifically the changes in milliampere-seconds (mAs) required when the Source-to-Image Distance (SID) is altered.
According to the inverse square law of radiation, the intensity of radiation at a given distance from the point source is inversely proportional to the square of the distance.
To maintain the same image quality (exposure), the mAs must be adjusted when the SID changes.
The original conditions were 20 mAs at 60" SID.
To find the new mAs for a 48" SID, we can use the direct square law formula:
mAs1 / (SID1)2 = mAs2 / (SID2)2, where mAs1 and SID1 are the original mAs and SID, and mAs2 and SID2 are the new values to be calculated.
Plugging in the values; 20 mAs / (60")2 = mAs2 / (48")2, we can solve for mAs2.
Thus, mAs2 = 20 mAs * (48/60)2
= 20 mAs * (0.8)2
= 20 mAs * 0.64
= 12.8 mAs.
Therefore, to produce a similar radiograph at 48" SID, 12.8 mAs is needed.