Final answer:
To calculate the new mAs value when changing the SID and adding a grid, use the grid factor for the 12:1 grid, and apply the inverse square law to adjust for the change in distance. After performing the calculations, you will find the new mAs value needed for a satisfactory radiograph.
Step-by-step explanation:
The student's question involves the calculation of a new milliampere-seconds (mAs) value when changing the source-to-image distance (SID) from 72 inches to 40 inches and adding a 12:1 ratio grid in radiography. The use of mAs and grids is essential in getting a proper exposure for a radiograph. When distance is decreased and a grid is added, the mAs must be adjusted to maintain image quality.
To calculate the new mAs setting, we use the grid conversion factor for the 12:1 grid and the inverse square law. First, find the grid factor for a 12:1 ratio grid, which is typically around 5. Then, apply the inverse square law to account for the distance change. The formula for the inverse square law states that intensity is inversely proportional to the square of the distance (I1/I2 = (D2/D1)^2).
Initial mAs without grid = 8 mAs
Distance change: 72 in. to 40 in.
Grid factor for 12:1 grid ≈ 5
Calculating the new mAs:
I1/I2 = (D2/D1)^2
I2 = I1 * (D1/D2)^2
New mAs without grid factor = 8 mAs * (72/40)^2
Adjust for grid factor: New mAs with grid = New mAs without grid * Grid factor
New mAs with grid ≈ (New mAs without grid) * 5
After performing the calculations, you will arrive at the new mAs value needed to produce a satisfactory radiograph with the changed parameters.