Which statement correctly describes how z-axis magnification depends on SID and OID and its effect on spatial resolution and dose?

• A. Magnification Increases With Larger SID
• B. Magnification Does Not Depend On SID Or OID
• C. Magnification Increases With Smaller SID and Larger OID; Higher Magnification Can Degrade Spatial Resolution and Increase Patient Dose
• D. Magnification Improves Spatial Resolution

Answer: (C)

Geometric magnification in radiography comes from the divergent x-ray beam. The amount the image is magnified depends on how far the object is from the source and how far it is from the image receptor. If you move the object closer to the source or farther from the receptor (smaller SID or larger OID), the image of the object on the detector grows larger. The magnification factor increases as SID decreases or OID increases.

When magnification is higher, geometric unsharpness (blur) becomes more pronounced along the z-axis because the finite size of the focal spot blurs edges more as rays diverge over a longer distance. That reduction in sharpness means spatial resolution is degraded.

Dose considerations follow the inverse-square law: decreasing SID increases the beam intensity at the patient, raising the entrance skin dose. So higher magnification, which often accompanies a smaller SID and/or larger OID, tends to increase the patient dose while reducing spatial resolution.