Define Source-to-Image Distance (SID) and explain its influence on magnification and dose.

• A. SID is the distance from the patient to the detector; increasing SID reduces magnification.
• B. SID is the distance from the X-ray source to the image receptor; increasing SID reduces magnification and decreases dose.
• C. SID is the distance from the X-ray source to the patient; increasing SID increases magnification.
• D. SID is the distance between the patient and the X-ray source; decreasing SID reduces dose.

Answer: (B)

Source-to-image distance is the distance from the X-ray tube (the source) to the image receptor. As this distance increases, the X-ray beam spreads out less when it hits the receptor, which lowers geometric magnification because the projection from object to receptor becomes closer to true size. The magnification factor can be described as SID divided by the distance from the source to the object, so increasing SID (with the object-to-receptor distance held constant) reduces magnification. Regarding dose, the beam’s intensity at the patient decreases with distance due to the inverse square law, so a longer SID tends to reduce the dose per image if exposure factors are not increased. The other statements either misdefine SID or imply the wrong direction for magnification or dose changes.