SI Joints AP Axial is a radiography positioning projection of the Pelvis. AP axial bilateral SI joints. CR 30-35° cephalad. SID 40". The centering point is located approximately 2 inches superior to the pubic symphysis on the midline (~2 inches inferior to the asis).. The central ray is 30-35 degrees cephalad (30° male, 35° female), centered to midline at approximately 2 inches above the pubic symphysis.. Image-quality criteria include both si joints shown symmetrically without rotation., si joint spaces clearly visualized.. Standard exposure ranges from 80 to 90 kVp, 15 to 25 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Both sacroiliac joints bilaterally.
- SI joint articulation surfaces.
- Adjacent sacral ala and iliac bone margins.
- Symmetry of sacroiliac articulation.
- Relation to lumbosacral junction.
Patient preparation
- Verify patient identity.
- Position patient supine on the table.
- Remove radiopaque objects from pelvis and lower spine.
- Align patient midline to centerline of table.
- Internally rotate feet approximately 10-15 degrees to align SI joint surfaces with the IR.
- Suspend respiration during exposure.
Position & centering point
Approximately 2 inches superior to the pubic symphysis on the midline (~2 inches inferior to the ASIS).
Central ray
30-35 degrees cephalad (30° male, 35° female), centered to midline at approximately 2 inches above the pubic symphysis.
Exposure / technique
- kVp
- 80–90
- mAs
- 15–25
- SID
- 40" (102 cm)
- Notes
- Moderate exposure. CR perpendicularity is critical for symmetric joint visualization.
Image-quality criteria
- Both SI joints shown symmetrically without rotation.
- SI joint spaces clearly visualized.
- No rotation of pelvis.
- Sacroiliac articulations centered on IR.
- Pelvis not tilted.
- Equal distance from sacrum midline to each SI joint.
Common errors / ARRT traps
- 1 Pelvic rotation asymmetrically demonstrates SI joints.
- 2 Feet not internally rotated obscures joint articulation.
- 3 CR not perpendicular elongates or foreshortens joints.
- 4 Pelvis tilted by improper leg positioning.
- 5 Overrotation of feet obscures far-side joint.
Clinical indications
- Suspected sacroiliitis from inflammatory arthropathy.
- Evaluation of ankylosing spondylitis progression.
- SI joint pain etiology assessment.
- Posttrauma SI joint assessment.
Aligned to the 2025 ARRT Content Specifications.