If a grid with an 8:1 ratio is added to an exposure of 90 kVp and 4 mAs, what will the new mAs be?

The introduction of a grid to an imaging technique increases the amount of mAs required to achieve the desired image density due to the grid's ability to absorb some of the primary radiation. The grid ratio, which in this case is 8:1, signifies that the grid can substantially reduce the amount of scattered radiation that reaches the image receptor. However, to compensate for this increased absorption, the mAs must be adjusted.

In typical practice, for an 8:1 grid ratio, a rule of thumb is to multiply the existing mAs by a factor of 4. This adjustment ensures that sufficient radiation reaches the detector for adequate image formation.

Starting with the original exposure of 4 mAs, the calculation for the new mAs after adding the grid would be:

4 mAs × 4 (the increase factor for an 8:1 grid) = 16 mAs.

Hence, when the grid is introduced, the new mAs becomes 16, facilitating optimal image quality while compensating for the increased absorption by the grid.