For a patient with an ASIS-to-tabletop distance greater than 24 cm, how should the central ray be directed for the AP knee projection?

• A. Perpendicular to the table
• B. 5 degrees cephalad
• C. 5 degrees caudad
• D. 10 degrees cephalad

Answer: (B)

When taking an AP knee radiograph, the central ray is angled to align with the knee’s joint line so the joint space is demonstrated accurately and without distortion. In patients with greater body habitus (ASIS-to-tabletop distance over 24 cm), the leg sits at a slight tilt due to the pelvis and thigh length. A small cephalad tilt of the CR by about 5 degrees compensates for that tilt, helping the beam run parallel to the tibial plateau and giving a true view of the knee joint.

Directing the beam perpendicular to the table would miss that compensatory alignment in a larger patient, potentially distorting the joint. A caudad angle would tilt the beam away from the joint line, and a 10-degree cephalad angle is usually more than necessary, risking overcorrection and altered projection. Therefore, a 5-degree cephalad angle is the appropriate adjustment for this scenario.