Which statement correctly describes AP knee projection when imaging a large patient?

Which statement correctly describes AP knee projection when imaging a large patient?

• A. 0.5 inch below the patellar apex
• B. 1/4 inch below the apex
• C. 2 inches below the patella
• D. Central ray angle varies with pelvic measurement and may be adjusted

Answer: (D)

AP knee imaging in a large patient requires adjusting the central ray angle based on pelvic width or measurement. The size and shape of the pelvis can tilt the femur relative to the knee, so a fixed vertical CR position may miscenter or distort the knee joint. By evaluating the pelvis and varying the CR angle accordingly, you can center the knee joint, keep the joint space accurately demonstrated, and ensure the femoral condyles are superimposed. Fixed placements that don’t account for body habitus don’t reliably produce an true AP projection in larger patients, making an adjustable central ray angle the correct approach.

AP knee imaging in a large patient requires adjusting the central ray angle based on pelvic width or measurement. The size and shape of the pelvis can tilt the femur relative to the knee, so a fixed vertical CR position may miscenter or distort the knee joint. By evaluating the pelvis and varying the CR angle accordingly, you can center the knee joint, keep the joint space accurately demonstrated, and ensure the femoral condyles are superimposed. Fixed placements that don’t account for body habitus don’t reliably produce an true AP projection in larger patients, making an adjustable central ray angle the correct approach.