What is the effect of increasing source-to-image distance (SID) on receptor exposure?

What is the effect of increasing source-to-image distance (SID) on receptor exposure?

• A. Increases
• B. No change
• C. Variable
• D. Decreases

Answer: (D)

Increasing the distance between the X-ray source and the image receptor reduces the amount of radiation that reaches the receptor because of the inverse square law. As SID grows, the beam spreads over a larger area, so the photon intensity per unit area on the receptor drops. The exposure at the receptor is proportional to 1/SID^2, so doubling SID cuts exposure to about one quarter, tripling it to about one ninth. To keep receptor exposure the same when SID increases, you would raise the exposure factors (primarily mAs) accordingly.

Increasing the distance between the X-ray source and the image receptor reduces the amount of radiation that reaches the receptor because of the inverse square law. As SID grows, the beam spreads over a larger area, so the photon intensity per unit area on the receptor drops. The exposure at the receptor is proportional to 1/SID^2, so doubling SID cuts exposure to about one quarter, tripling it to about one ninth. To keep receptor exposure the same when SID increases, you would raise the exposure factors (primarily mAs) accordingly.