How does source-to-image distance (SID) influence beam intensity and image geometry?

• A. Intensity falls with the square of the distance; increasing SID reduces exposure and reduces magnification, improving image sharpness.
• B. SID does not affect beam intensity or image geometry.
• C. Increasing SID increases exposure and magnification.
• D. Increasing SID reduces exposure but increases magnification.

Answer: (A)

The main idea is that how far the image receptor is from the x‑ray source changes both how much beam energy reaches the detector and how the image is formed geometrically. The beam intensity at the image receptor follows the inverse square law: as SID grows, the detected intensity drops with the square of the distance. If you keep the tube settings the same, increasing SID lowers the exposure at the receptor. To keep the image receptor exposure the same, you’d need to raise the exposure factors (mA or exposure time) accordingly.

Geometrically, magnification and blur depend on the distances among source, object, and detector. Increasing SID, with the object position fixed, reduces magnification and geometric blur, so the image sharpness improves. This happens because the projection geometry makes the object appear closer to its real size when the source is farther away.

So, increasing SID reduces intensity at the detector (unless exposure is increased to compensate) and reduces magnification, leading to a sharper image.