Which combination of object-to-image distance (OID) and source-to-image distance (SID) would produce the highest spatial resolution?

• A. 4 inch (10 cm) OID and 40 inch (102 cm) SID
• B. 3 inch (8 cm) OID and 40 inch (102 cm) SID
• C. 4 inch (10 cm) OID and 72 inch (183 cm) SID
• D. 1 inch (3 cm) OID and 72 inch (183 cm) SID

Answer: (D)

Spatial resolution in radiography is governed by geometric unsharpness, which grows with the object-to-image distance (OID) and shrinks with a longer source-to-image distance (SID). The blur can be estimated by U ≈ FSS × (OID / SID), so minimizing OID while maximizing SID reduces geometric blur and sharpens the image.

Looking at the options, the smallest OID is 1 inch and the longest SID is 72 inches, giving the smallest OID/SID ratio and thus the least geometric blur. Numerically, the ratios are 4/40, 3/40, 4/72, and 1/72; the smallest value is 1/72, which corresponds to the 1 inch OID with 72 inch SID. This combination provides the highest spatial resolution.