For an AP oblique projection of the sacroiliac joints with the patient in an RPO position, where should the central ray be placed?

• A. Parallel to the ASIS
• B. 1 inch medial to the ASIS and perpendicular to the IR
• C. 1 inch lateral to the ASIS
• D. Perpendicular toward the midline at L4

Answer: (B)

For an AP oblique view of the sacroiliac joints, the central ray is positioned to hit the SI joint itself by centering about 1 inch medial to the ASIS on the side being examined and making the ray perpendicular to the image receptor. In an RPO position, the patient is rotated enough (about 25–30 degrees) to place the sacroiliac joint in profile, and directing the beam at this medial-to-ASIS point ensures the joint area is centered and imaged without distortion. If the ray were parallel to the ASIS, directed laterally, or aimed toward a vertebral level like L4, the SI joint would not be centered or properly visualized, resulting in poor visualization of the joint space.