What is the recommended source-to-image distance (SID) for an anteroposterior (AP) knee radiograph?

• A. 40 inches (102 cm)
• B. 72 inches (183 cm)
• C. 30 inches (76 cm)
• D. 50 inches (127 cm)

Answer: (A)

Source-to-image distance changes how large and how sharp the knee appears on the image. For an AP knee radiograph, about 40 inches (102 cm) is the standard SID because the knee sits close to the detector, and this distance gives a good balance between minimizing magnification and keeping the exposure practical. If you use a much shorter distance, the knee would be more magnified and the margins less sharp, making it harder to assess the joint properly. If you go much longer than 40 inches, magnification decreases further, but you’d need more exposure to maintain the same image brightness, and positioning becomes more awkward for routine knee imaging. So the 40-inch distance is the typical compromise that provides adequate detail without unnecessarily increasing dose or complicating setup.