What is the recommended source-to-image distance (SID) for the tangential projection of the knee using the Settegast method?

• A. 40 inches (102 cm)
• B. 60 inches (152 cm)
• C. 72 inches (183 cm)
• D. 30 inches (76 cm)

Answer: (A)

The key idea is imaging geometry for a tangential knee view. When performing the Settegast tangential projection, the knee is flexed so the patellofemoral joint is imaged tangentially, and the distance from the tube to the image receptor is chosen to place the patella cleanly near the center with minimal distortion.

A source-to-image distance of about 40 inches (102 cm) is standard for this tangential knee view. This distance provides a practical setup that keeps the patellofemoral joint centered on the image and allows the beam to travel at an appropriate angle to trace the patella’s contours without excessive magnification or distortion. Using a shorter distance would increase magnification and distortion of the patellar edge, while a much longer distance would complicate positioning and alignment without offering meaningful advantages for this view.