If you decrease the source-to-image distance (SID), what happens to spatial resolution?

If you decrease the source-to-image distance (SID), what happens to spatial resolution?

• A. Spatial resolution decreases
• B. Spatial resolution increases
• C. No change
• D. Penumbra disappears

Answer: (A)

Decreasing the source-to-image distance makes the geometric blur around edges larger, so the image becomes less sharp. This happens because the finite size of the x-ray focal spot means rays from different parts of the spot project edges at slightly different angles onto the image receptor; when you bring the source closer, those angles spread more and blur increases (penumbra grows). So spatial resolution, which is about how clearly fine details are shown, decreases. The alternative outcomes don’t fit: shrinking the distance doesn’t make the image sharper or leave sharpness unchanged, and the edge blur (penumbra) does not disappear—it becomes more pronounced as SID gets smaller.

Decreasing the source-to-image distance makes the geometric blur around edges larger, so the image becomes less sharp. This happens because the finite size of the x-ray focal spot means rays from different parts of the spot project edges at slightly different angles onto the image receptor; when you bring the source closer, those angles spread more and blur increases (penumbra grows). So spatial resolution, which is about how clearly fine details are shown, decreases.

The alternative outcomes don’t fit: shrinking the distance doesn’t make the image sharper or leave sharpness unchanged, and the edge blur (penumbra) does not disappear—it becomes more pronounced as SID gets smaller.