Which statement is true about the central ray angle for the AP knee radiograph?

• A. It is always perpendicular to the table
• B. It varies based on the pelvic measurement
• C. It varies based on the pelvic measurement
• D. It is directed 5 degrees cephalad for all patients

Answer: (C)

The central ray angle for an AP knee radiograph isn’t fixed; it’s adjusted to fit the patient’s anatomy. Because the pelvis and femur can sit at different angles from person to person, the way the knee sits relative to the table changes. Using pelvic measurements helps determine how much the CR should be angled so the knee joint line is captured with the joint spaces open and the projection isn’t distorted. That’s why this angle varies with pelvic measurement. It isn’t always perpendicular to the table, and there isn’t a universal 5-degree cephalad rule for everyone.