Question

An exposure was made of a part using 300 mA and 0.06 second with a 200-speed film-screen combination. An additional radiograph is requested using a 400-speed system to reduce motion unsharpness. Using 400 mA, all other factors remaining constant, what should be the new exposure time?

22 ms

Answer 1

The new exposure time when using a 400-speed system and 400 mA, while maintaining the same image density, should be 45 milliseconds.

Step-by-step explanation:

The student's question involves calculating the new exposure time for an x-ray when changing from a 200-speed film-screen combination to a 400-speed system and adjusting the current from 300 mA to 400 mA. Since the speed of the film-screen system is doubled, the amount of radiation needed for adequate exposure is halved. This means that the exposure time should also be halved to maintain the same image density. Keeping all other factors constant and using inverse square law and the concept of exposure maintenance formula (mAs), if the original time was 0.06 seconds at 300 mA, the new time with 400 mA would be calculated as follows:

Original mAs = 300 mA × 0.06 s = 18 mA

To achieve the same mAs with 400 mA:

18 mAs = 400 mA × New Time

New Time = 18 mAs / 400 mA = 0.045 s or 45 ms

Therefore, the new exposure time using a 400-speed system and 400 mA to reduce motion unsharpness should be 45 ms.