Final answer:
To achieve a similar radiograph at a 60" SID compared to the original 72" SID with 100 mAs, the Inverse Square Law must be applied. The calculation shows that 5.76 mAs is required to maintain image quality at the reduced distance.
Step-by-step explanation:
The question deals with the concept of radiographic exposure and how it is affected by changes in the Source-to-Image Distance (SID). In radiography, the exposure (amount of radiation reaching the film) is determined by factors like milliampere-seconds (mAs) and SID, following the Inverse Square Law. The question requires a calculation to maintain image quality when changing the SID. According to the Inverse Square Law, the intensity of radiation inversely varies with the square of the distance. Therefore, when the distance is reduced, the mAs must be reduced proportionately to maintain the same radiographic exposure. Specifically:
mAs1 / mAs2 = (SID2 / SID1)^2
Substituting the given values:
100 mAs / mAs2 = (60" / 72")^2
Simplifying:
100 mAs / mAs2 = (5 / 6)^2
100 mAs / mAs2 = 25 / 36
mAs2 = (100 mAs * 36) / 25
mAs2 = 144 mAs / 25
mAs2 = 5.76 mAs
Therefore, 5.76 mAs is needed to make a similar radiograph at a 60" SID.