Final answer:
To find the new exposure time for a radiograph, you must calculate the product of current and time, and adjust for changes in kV. The initial calculation assuming equal exposure yields 75 ms, but the increase in kV necessitates a further reduction in time.
Step-by-step explanation:
The student is asking how to calculate the new exposure time for a radiograph when the current settings cause motion unsharpness. The original exposure was made using 300 mA, 0.1 second, and 75 kV. The new settings will be 86 kV and 400 mA. To maintain the same x-ray exposure (which is proportional to the product of current and time), one must solve the equation 300 mA × 0.1 s = 400 mA × t, where t is the new exposure time. Solving for t gives t = (300 mA × 0.1 s) / 400 mA, which equals 0.075 seconds or 75 ms. However, because we also increased the kV from 75 to 86, the total x-ray output will be increased, so the time should be reduced further from 75 ms. The precise adjustment requires information on how the x-ray output is affected by the change in kV, which requires the application of the inverse square law and the fact that the x-ray intensity is proportional to the square of the voltage ratio. A professional in the field would typically use established tables or formulas, as the relationship is not linear.