Final answer:
To calculate the new mAs when a 12:1 grid is added to an X-ray imaging setup with an initial setting of 90 kV and 3 mAs, multiply the original mAs by the grid factor, which is typically 5 for a 12:1 grid, resulting in a new mAs value of 15 mAs.
Step-by-step explanation:
The student's question is about adjusting the milliampere-seconds (mAs) when switching from a non-grid exposure to using a grid in X-ray imaging. Specifically, they ask about the new mAs value needed when a 12:1 grid is added to an existing setup of 90 kV and 3 mAs.
When adding a grid, additional mAs is required to maintain image quality because a grid absorbs more of the scattered radiation. There is a specific factor called the 'grid conversion factor' or 'Bucky factor' that's used for calculating the new mAs when a grid is added. For a 12:1 grid, this factor is typically around 5. Therefore, to adjust the mAs for a 12:1 grid, you would multiply the original mAs (3 mAs in this case) by the grid factor:
Adjusted mAs = Original mAs × Grid conversion factor
Adjusted mAs = 3 mAs × 5
Adjusted mAs = 15 mAs
Therefore, the new milliampere-seconds value would be 15 mAs when a 12:1 grid is added.