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 on the midline, about 2 inches inferior to the ASIS. The central ray is 30-35 degrees cephalad (30° male, 35° female), centered to midline about 2 inches inferior to the ASIS. Image-quality criteria include both SI joints shown symmetrically without rotation, SI joint spaces clearly visualized. A sample technique is 90 kVp with DR 14 or CR 28 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.
- Keep the legs extended, with support under the knees for comfort.
- Suspend respiration during exposure.
Position & centering point
On the midline, about 2 inches inferior to the ASIS.
Central ray
30-35 degrees cephalad (30° male, 35° female), centered to midline about 2 inches inferior to the ASIS.
Exposure / technique
- kVp
- 90
- mAs
- DR 14 / CR 28
- SID
- 40" (102 cm)
- Notes
- Bontrager: males generally need about 30 degrees cephalad and females 35, with an increase in the lumbosacral curve. Sample technique: DR 14 / CR 28 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
- Both SI joints shown symmetrically without rotation.
- SI joint spaces clearly visualized.
- No rotation of pelvis.
- Sacroiliac articulations centered on IR.
- Spinous process of L5 in the center of the vertebral body.
- Equal distance from sacrum midline to each SI joint.
Common errors
- 1 Pelvic rotation asymmetrically demonstrates SI joints.
- 2 CR angle not matched to the patient: about 30 degrees for males and 35 for females.
- 3 CR not directed to the midline about 2 inches below the level of the ASIS.
Clinical indications
- Pathology of the SI joint, including fracture and joint dislocation or subluxation.
- Evaluation of ankylosing spondylitis progression.
- Posttrauma SI joint assessment.