Which distance is measured from the focal spot to the image receptor?

Which distance is measured from the focal spot to the image receptor?

• A. SID
• B. OID
• C. SOD
• D. FSS

Answer: (A)

Source-to-Image Distance (SID) is the distance from the focal spot to the image receptor. This distance governs image geometry: increasing SID reduces magnification and improves sharpness because the X-ray beams diverge less before reaching the receptor. However, a longer SID also lowers the beam intensity at the receptor, so exposure technique (mA·s) must be increased to maintain receptor exposure. The other terms describe different parts of the setup—object-to-image distance and source-to-object distance relate to magnification in other ways, and focal spot size affects geometric blur—so the direct path from the focal spot to the receptor is specifically SID.

Source-to-Image Distance (SID) is the distance from the focal spot to the image receptor. This distance governs image geometry: increasing SID reduces magnification and improves sharpness because the X-ray beams diverge less before reaching the receptor. However, a longer SID also lowers the beam intensity at the receptor, so exposure technique (mA·s) must be increased to maintain receptor exposure. The other terms describe different parts of the setup—object-to-image distance and source-to-object distance relate to magnification in other ways, and focal spot size affects geometric blur—so the direct path from the focal spot to the receptor is specifically SID.