Which factor does not directly affect geometric unsharpness according to the standard formula U = FSS × (OID / SOD)?

Which factor does not directly affect geometric unsharpness according to the standard formula U = FSS × (OID / SOD)?

• A. Focal spot size
• B. Detector pixel size
• C. OID
• D. SOD

Answer: (B)

Geometric unsharpness comes from the finite size of the focal spot and how the X-ray beam projects the object onto the detector. The formula U = FSS × (OID / SOD) shows that blur grows with the focal spot size and with the distance from the object to the image (OID), and it shrinks when the distance from the source to the object (SOD) increases. In other words, a larger focal spot or a larger OID increases blur, while a larger SOD (source-to-object distance) reduces it. Detector pixel size isn’t part of that geometric blur equation, so it doesn’t directly affect geometric unsharpness. It does influence how finely the final image can be sampled and resolved digitally, but the actual geometric blur value comes from FSS, OID, and SOD.

Geometric unsharpness comes from the finite size of the focal spot and how the X-ray beam projects the object onto the detector. The formula U = FSS × (OID / SOD) shows that blur grows with the focal spot size and with the distance from the object to the image (OID), and it shrinks when the distance from the source to the object (SOD) increases. In other words, a larger focal spot or a larger OID increases blur, while a larger SOD (source-to-object distance) reduces it.

Detector pixel size isn’t part of that geometric blur equation, so it doesn’t directly affect geometric unsharpness. It does influence how finely the final image can be sampled and resolved digitally, but the actual geometric blur value comes from FSS, OID, and SOD.