A lateral radiograph of the cervical spine was made at 40 inches using 3 mAs. What should be the new mAs if the distance is increased to 72 inches?

To determine the new mAs when changing the distance from 40 inches to 72 inches, the relationship between distance and exposure needs to be understood. This relationship is governed by the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source.

When the distance increases, the exposure must also increase to maintain the same level of image quality and to properly penetrate the subject. The formula used to adjust the mAs when changing distance is:

New mAs = Old mAs x (New Distance / Old Distance)²

Using the provided figures:

• Old mAs = 3

• Old Distance = 40 inches

• New Distance = 72 inches

Calculating the new mAs:

New mAs = 3 mAs x (72 inches / 40 inches)²

New mAs = 3 mAs x (1.8)²

New mAs = 3 mAs x 3.24

New mAs = 9.72 mAs, which is approximately 10 mAs when rounded.

This calculation indicates that to maintain image quality with the increased distance, the new mAs should indeed be 10 mAs. This