Which statement about source-to-image distance (SID) and size distortion is correct?

• A. Size distortion decreases with a longer SID
• B. Size distortion increases with a longer SID
• C. Size distortion is independent of SID
• D. Size distortion varies randomly with SID

Answer: (A)

Size distortion, or magnification, in radiography is governed by the geometry between the X-ray source, the object, and the image receptor. The magnification that appears on the image depends on how far the receptor is from the source relative to how far the source is from the object. If you increase the source-to-image distance while keeping the object distance fixed, the image magnification decreases. Mathematically, magnification M is SID divided by SOD (or SID divided by SID minus OID). As SID grows, SOD grows too, but the ratio moves closer to 1, so the image size more closely matches the actual object size. A practical example shows that with a fixed object-to-image distance, longer SID reduces magnification — the projection on the receptor becomes less enlarged. Therefore, size distortion decreases with a longer SID.