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

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

• A. 0.5 inch (1 cm) below the apex of the patella
• B. Directly through the patella
• C. At the tibial tuberosity
• D. 1.5 inches below the patellar apex

Answer: (A)

Centering the central ray at the knee joint line is essential for an AP knee radiograph. Directing the CR about 0.5 inch (1 cm) below the apex of the patella places the knee joint—the junction between the femur and tibia—squarely in the image center, ensuring the joint spaces and alignment are well demonstrated. Aiming through the patella would misalign the joint line and likely miss the true joint space, while directing at the tibial tuberosity or far below the patellar apex would shift the anatomy out of the view and distort the image. The CR should be perpendicular to the knee joint line with the knee extended for an accurate AP projection.

Centering the central ray at the knee joint line is essential for an AP knee radiograph. Directing the CR about 0.5 inch (1 cm) below the apex of the patella places the knee joint—the junction between the femur and tibia—squarely in the image center, ensuring the joint spaces and alignment are well demonstrated. Aiming through the patella would misalign the joint line and likely miss the true joint space, while directing at the tibial tuberosity or far below the patellar apex would shift the anatomy out of the view and distort the image. The CR should be perpendicular to the knee joint line with the knee extended for an accurate AP projection.