In focal spot blur calculations, SOD stands for which distance?

• A. Source-to-image distance
• B. Source-to-object distance
• C. Object-to-image distance
• D. Source-to-detector distance

Answer: (B)

In focal spot blur calculations, the distance from the X-ray tube’s focal spot to the part of the patient (the object) being imaged is what SOD represents. This Source-to-Object Distance is a key geometric parameter because it, together with how far the image receptor is from the source, determines magnification and how much the finite size of the focal spot contributes to blur on the image. In practical terms, magnification is governed by SID and SOD (SID/SOD), linking these distances to image sharpness. The other options describe distances from the source to the image receptor or from the object to the image receptor, not from the source to the object, so they are not what SOD stands for.