Increasing the source-to-image distance (SID) has what effect on receptor exposure?

Increasing the source-to-image distance (SID) has what effect on receptor exposure?

• A. Increases receptor exposure
• B. Has no effect on receptor exposure
• C. Increases patient dose
• D. Decreases receptor exposure

Answer: (D)

Increasing the distance between the X-ray source and the image receptor highlights how beam intensity changes with distance, described by the inverse square law. As SID gets larger, the beam spreads over a bigger area, so the number of photons reaching a given spot on the receptor drops proportional to 1/distance^2. The result is lower receptor exposure with a longer SID. If you needed to keep receptor exposure the same at a greater SID, you would have to raise exposure factors (more mA or longer exposure time or higher kVp), which would increase patient dose. The direct effect of increasing SID is a decrease in receptor exposure, not an increase, and it certainly isn’t “no effect.”

Increasing the distance between the X-ray source and the image receptor highlights how beam intensity changes with distance, described by the inverse square law. As SID gets larger, the beam spreads over a bigger area, so the number of photons reaching a given spot on the receptor drops proportional to 1/distance^2. The result is lower receptor exposure with a longer SID. If you needed to keep receptor exposure the same at a greater SID, you would have to raise exposure factors (more mA or longer exposure time or higher kVp), which would increase patient dose. The direct effect of increasing SID is a decrease in receptor exposure, not an increase, and it certainly isn’t “no effect.”