If SID is doubled, what may be said about receptor exposure?

If SID is doubled, what may be said about receptor exposure?

• A. Receptor exposure doubles
• B. Receptor exposure is reduced by one half
• C. Receptor exposure is reduced by new mAs^2
• D. Receptor exposure is reduced to one fourth

Answer: (D)

The key idea is the inverse square law for X-ray exposure: receptor exposure is proportional to the incident intensity, which varies as 1 over the SID squared. Doubling the source-to-image distance makes the distance four times larger when you square it, so the exposure at the receptor drops to 1/4 of its original value. Therefore, receptor exposure is reduced to one fourth. If you wanted to keep the same exposure with a doubled SID, you’d need to increase the mAs by a factor of 4 (since exposure ≈ mAs / SID^2).

The key idea is the inverse square law for X-ray exposure: receptor exposure is proportional to the incident intensity, which varies as 1 over the SID squared. Doubling the source-to-image distance makes the distance four times larger when you square it, so the exposure at the receptor drops to 1/4 of its original value. Therefore, receptor exposure is reduced to one fourth.

If you wanted to keep the same exposure with a doubled SID, you’d need to increase the mAs by a factor of 4 (since exposure ≈ mAs / SID^2).