Where should the central ray be directed for an anteroposterior (AP) knee radiograph?

For an anteroposterior (AP) knee radiograph, the central ray should be directed 0.5 inch (1 cm) below the apex of the patella. This positioning is crucial as it ensures that the x-ray beam is centered over the joint space of the knee. The apex of the patella serves as a reliable anatomical landmark, and directing the beam just inferior to this point helps to visualize the knee joint clearly, including the femoral condyles, tibial plateau, and the patellofemoral joint without excessive distortion.

Choosing this point also accommodates for variations in anatomy and ensures that both the upper and lower parts of the knee joint are adequately included in the image. Proper centering contributes to better diagnostic quality of the radiographs by minimizing distortion and preventing artifacts that may arise from misalignment.