Poor spatial resolution is caused by unacceptable levels of

• A. Focal spot size
• B. Image noise
• C. Penumbra
• D. Patient motion blur

Answer: (C)

Spatial resolution is about how well an image can display fine detail and distinct edges. In radiography, edge sharpness is reduced when there is geometric unsharpness, known as penumbra—the soft, partially exposed region that sits around object edges. This happens because the X-ray source has a finite size, so rays from different parts of the focal spot project slightly different shadows onto the image. The overlapping shadows create a gradual transition from light to dark rather than a crisp boundary, making small details harder to resolve. If penumbra is large, edges blur and the image loses sharpness, which is why unacceptable levels of penumbra lead to poor spatial resolution. Image noise contributes to graininess in gray levels but doesn’t blur edges in the same geometric way, and motion blur is a different mechanism that also reduces sharpness but isn’t the defining term for edge unsharpness described here.