To maintain original receptor exposure when increasing SID from 40 inches to 48 inches and changing grid ratio from 12:1 to 8:1, what should the new mAs be?

When adjusting the source-to-image distance (SID) and grid ratio, calculating the new milliampere-seconds (mAs) is essential to maintain the same receptor exposure.

Increasing the SID from 40 inches to 48 inches necessitates an increase in mAs to compensate for the increase in distance because X-ray intensity decreases with distance. The formula for adjusting mAs with distance is based on the inverse square law, which states that if the distance is increased, the mAs should also be increased proportionally to maintain exposure. Specifically, for every doubling of distance, the required mAs must quadruple to maintain the same exposure.

In this case, since the distance increases from 40 to 48 inches, a factor can be calculated using the square of the ratio of the distances. The adjustment factor is derived as follows:

[ \text{New mAs} = \text{Old mAs} \times \left(\frac{\text{New SID}}{\text{Old SID}}\right)^2 ]

Next, when changing from a grid ratio of 12:1 to 8:1, the lower grid ratio is more efficient at allowing more of the primary beam to reach the receptor, which means that less mAs