Which of the following variables, when increased, is associated with increased spatial resolution?

• A. filament size
• B. anode angle
• C. object-to-image distance (OID)
• D. source-to-object distance (SOD)

Answer: (D)

Spatial resolution improves when geometric unsharpness is reduced. In radiography, blur at the image plane depends on the focal spot size and the geometry of distances from source to object and from object to image receptor. If you increase the distance from the X‑ray source to the object (while keeping the object-to-image distance constant, so the overall source-to-image distance grows), the geometric magnification decreases. This lowers the unsharpness term because it is proportional to the focal spot size times the ratio of the object-to-image distance to the source-to-image distance. In short, increasing source-to-object distance reduces magnification blur and sharpens detail.

Increasing object-to-image distance would increase blur and degrade resolution, and enlarging the focal spot (filament) size or changing the anode angle tends to increase geometric blur as well, reducing sharpness. So the distance that, when increased, enhances spatial resolution is the source-to-object distance.