SI Joints AP Oblique (RPO) is a radiography positioning projection of the Pelvis. AP oblique SI joints, RPO. SID 40". CR perpendicular to IR. The centering point is located at the medial border of the demonstrated si joint (the one farthest from the ir / elevated side), approximately 1 inch medial to the elevated asis.. The central ray is perpendicular to ir, centered to medial border of demonstrated si joint.. Image-quality criteria include demonstrated si joint shown open and free of superimposition., si joint articulation surface clearly visualized.. Standard exposure ranges from 80 to 90 kVp, 15 to 25 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Demonstrated SI joint articulation in profile.
- Medial iliac margin adjacent to SI joint.
- Sacral ala near joint articulation.
- Superior and inferior SI joint margins.
- Relationship of SI joint to adjacent pubic bone.
Patient preparation
- Verify patient identity.
- Position patient supine on the table.
- Rotate patient 25-30 degrees (RPO for right SI joint).
- Left side of pelvis raised off the table.
- Support under right hip and pelvis for stability.
- Remove radiopaque objects from pelvis.
- Align patient with CR path to centering point.
- Suspend respiration during exposure.
Position & centering point
At the medial border of the demonstrated SI joint (the one farthest from the IR / elevated side), approximately 1 inch medial to the elevated ASIS.
Central ray
Perpendicular to IR, centered to medial border of demonstrated SI joint.
Exposure / technique
- kVp
- 80–90
- mAs
- 15–25
- SID
- 40" (102 cm)
- Notes
- Oblique angle 25-30 degrees. Centering accuracy critical for demonstrated joint.
Image-quality criteria
- Demonstrated SI joint shown open and free of superimposition.
- SI joint articulation surface clearly visualized.
- Iliac bone margin adjacent to joint delineated.
- Sacral ala near joint shown.
- No superimposition of contralateral anatomy.
- Joint centered on IR.
Common errors / ARRT traps
- 1 Insufficient oblique angle (less than 25°) overlaps joint spaces.
- 2 Excessive oblique angle (more than 30°) distorts joint anatomy.
- 3 CR not perpendicular projects joint out of true plane.
- 4 Centering off medial joint border causes off-axis positioning.
- 5 Patient rotation varies during long exposure.
Clinical indications
- Suspected sacroiliitis with need for joint detail.
- Evaluation of unilateral SI joint pathology.
- Assessment of SI joint fusion post-surgery.
- Inflammatory arthropathy progression evaluation.
Aligned to the 2025 ARRT Content Specifications.