What are the two effects of decreasing the source-to-image distance (SID)?

What are the two effects of decreasing the source-to-image distance (SID)?

• A. Increased spatial resolution
• B. Decreased spatial resolution and increased penumbra
• C. Decreased exposure to film
• D. Increased patient dose

Answer: (B)

Shorter source-to-image distance changes the projection geometry so the X-ray beam diverges more as it travels to the image receptor. That extra divergence broadens the geometric blur, enlarging the penumbra around edges. With a bigger penumbra, details become less distinct, so spatial resolution decreases. In other words, a shorter SID makes the image blurrier and edges less sharp. The other options either describe effects that aren’t about image sharpness (like increasing exposure or dose) or misstate the image quality outcome.

Shorter source-to-image distance changes the projection geometry so the X-ray beam diverges more as it travels to the image receptor. That extra divergence broadens the geometric blur, enlarging the penumbra around edges. With a bigger penumbra, details become less distinct, so spatial resolution decreases. In other words, a shorter SID makes the image blurrier and edges less sharp. The other options either describe effects that aren’t about image sharpness (like increasing exposure or dose) or misstate the image quality outcome.