Which effect does increasing the source-to-image distance (SID) have on imaging geometry?

Which effect does increasing the source-to-image distance (SID) have on imaging geometry?

• A. Increases magnification and blur.
• B. No effect on magnification.
• C. Reduces magnification and geometric blur, improving spatial resolution.
• D. Increases focal spot blur.

Answer: (C)

When the distance from the X-ray source to the image receptor is increased, the geometry of the image changes in two helpful ways. Magnification depends on the ratio of SID to SOD (image distance from source to receptor over source-to-object distance). If you keep the object’s position fixed and only increase SID, SOD increases as well, making the SID/SOD ratio smaller overall, so the image size moves closer to the actual object (less magnification). At the same time, geometric blur is proportional to f × (OID) / SID (where f is focal spot size and OID is the object-to-image distance). Increasing SID lowers this blur because the rays diverge less over the greater distance, sharpening the image. So increasing SID reduces both magnification and geometric blur, improving spatial resolution. (Note: to maintain image brightness, exposure often needs adjustment, due to the inverse square relationship.)

When the distance from the X-ray source to the image receptor is increased, the geometry of the image changes in two helpful ways. Magnification depends on the ratio of SID to SOD (image distance from source to receptor over source-to-object distance). If you keep the object’s position fixed and only increase SID, SOD increases as well, making the SID/SOD ratio smaller overall, so the image size moves closer to the actual object (less magnification). At the same time, geometric blur is proportional to f × (OID) / SID (where f is focal spot size and OID is the object-to-image distance). Increasing SID lowers this blur because the rays diverge less over the greater distance, sharpening the image. So increasing SID reduces both magnification and geometric blur, improving spatial resolution. (Note: to maintain image brightness, exposure often needs adjustment, due to the inverse square relationship.)