For the AP oblique projection of the sacroiliac joints in an RPO position, what is the correct orientation of the central ray?

• A. Angled 15 degrees cephalad
• B. Perpendicular to the plane of the body
• C. Angled 25-30 degrees caudal
• D. Horizontal

Answer: (B)

In an AP oblique projection of the sacroiliac joints with an RPO position, the central ray is directed perpendicular to the plane of the body. The patient is rotated about 25–30 degrees to bring the sacroiliac joints into a favorable oblique orientation, but the beam itself remains perpendicular to the body to avoid angular distortion of the joint spaces. This setup helps the x-ray beam pass through the joints in their true orientation, producing an image where the sacroiliac joint spaces can be accurately assessed. Angling the beam cephalad or caudal would tilt the projection and distort the joints, while a horizontal beam would not align with the oblique anatomy.