A radiograph acquired at 500 mA for 100 ms needs to be repeated at 400 ms. What new mA is required for the same receptor exposure?

To maintain the same receptor exposure when changing the time (milliseconds) of the exposure, you can use the relationship between milliampere (mA), exposure time (s), and the total exposure. The overall exposure is determined by the product of the mA and the duration of the exposure in seconds.

Initially, a radiograph is made at 500 mA for 100 ms (0.1 seconds). To calculate the exposure for this setting, you multiply the mA by the time:

500 mA * 0.1 s = 50 mAs (milliampere-seconds).

Now, if you need to repeat the radiograph at a new exposure time of 400 ms (0.4 seconds), you must keep the total exposure (mAs) the same to maintain the same receptor exposure. This means you must find the new mA value that will give you 50 mAs over a 0.4-second exposure.

Using the formula for mAs, rearranging gives:

mA = mAs / time in seconds.

Substituting the known values into this formula:

mA = 50 mAs / 0.4 s = 125 mA.

Thus, the new mA