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. A sample technique is 90 kVp with DR 16 or CR 36 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.
Patient preparation
- Verify patient identity.
- Position patient supine on the table.
- Rotate patient 25-30 degrees into RPO, which demonstrates the LEFT (elevated) SI joint.
- Left side of pelvis raised off the table.
- Support under the elevated left 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
- 90
- mAs
- DR 16 / CR 36
- SID
- 40" (102 cm)
- Notes
- Oblique angle 25-30 degrees. The CR may be angled 15-20 degrees cephalad to show the inferior part of the joint more clearly. Sample technique: DR 16 / CR 36 mAs at 90 kVp, 40 in SID, for a 17 cm part (Merrill's sample chart). Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Demonstrated SI joint shown open and free of superimposition.
- SI joint articulation surface clearly visualized.
- Bony margins and trabecular markings clear, with no motion.
- Sacral ala near joint shown.
- No superimposition of the ala of the ilium and sacrum on the open joint.
- Joint centered on IR.
Common errors
- 1 Inaccurate rotation (the book gives 25-30°): the ala of the ilium and the sacrum superimpose on the joint.
- 2 CR not directed 1 inch medial to the upside ASIS.
Clinical indications
- Pathology of the SI joint, including dislocation or subluxation.
- Evaluation of unilateral SI joint pathology.
- Bilateral study for comparison, using both obliques.