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

The recommended source-to-image distance (SID) for an anteroposterior (AP) knee radiograph is 40 inches (102 cm). This distance is chosen to optimize the balance between image detail and patient exposure to radiation.

At this SID, the X-ray beam can spread adequately over the knee joint while maintaining sufficient resolution of anatomical structures. A shorter SID may lead to increased magnification and decreased sharpness of the image, while a longer SID could reduce radiation exposure but also diminish image quality, particularly for structures that are relatively close together, such as the bones and soft tissues of the knee.

This standard distance helps ensure that the resulting radiographs clearly depict the joint space, bone contours, and any potential pathologies, which is critical for accurate diagnosis and treatment planning.