To reduce receptor exposure for a portable chest radiograph when technique cannot be increased, which action should be taken?

• A. Increase the exposure time
• B. Increase filtration
• C. Increase SID
• D. Decrease grid ratio

Answer: (C)

The key idea is that the beam’s intensity at the image receptor follows the inverse square law: as distance from the x‑ray source to the receptor increases, the exposure at the receptor drops rapidly. So when you can’t raise the technique (mA or kVp) to compensate, moving the receptor farther from the source—in other words, increasing the source-to-image-receptor distance—reduces the amount of radiation reaching the receptor. This is why increasing SID is the best way to lower receptor exposure in a portable chest study. For example, doubling the SID cuts receptor exposure to about one quarter, illustrating the effectiveness of distance on dose at the receptor. The other options either increase receptor exposure (longer exposure time, lower grid ratio) or have only a smaller, less reliable effect (filtration changes affect both patient and receptor dose and image quality).