Sacrum AP Axial is a radiography positioning projection of the Pelvis. Axial view of sacrum. CR 15° cephalad. SID 40". The centering point is located midline, 2 inches superior to the pubic symphysis. The central ray is 15° cephalad along the midline, entering 2 inches superior to the pubic symphysis. Image-quality criteria include sacral ala shown symmetrically without rotation, sacral foramina visible on both sides. A sample technique is 90 kVp with DR 14 or CR 28 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Entire sacrum from base to apex.
- Sacral ala bilaterally.
- Anterior sacral foramina.
- Coccyx as a rotation marker; it needs its own 10-degree caudad view.
- Sacroiliac joint articulation.
- Lumbosacral junction region.
Patient preparation
- Verify patient identity.
- Position patient supine on the table.
- Remove all radiopaque objects from pelvis and lower spine.
- Keep the legs extended.
- Place sandbag or pillow under knees if needed for comfort.
- Suspend respiration during exposure.
Position & centering point
Midline, 2 inches superior to the pubic symphysis.
Central ray
15° cephalad along the midline, entering 2 inches superior to the pubic symphysis.
Exposure / technique
- kVp
- 90
- mAs
- DR 14 / CR 28
- SID
- 40" (102 cm)
- Notes
- Grid required. CR may need 20 degrees cephalad with a greater posterior curvature or tilt of the sacrum and pelvis. 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
- Sacral ala shown symmetrically without rotation.
- Sacral foramina visible on both sides.
- Median sagittal crests and coccyx aligned with the symphysis pubis, showing no rotation.
- Sacrum centered on IR.
- No superimposition of pubic symphysis over sacrum.
- Sacral apex and foramina clearly demonstrated.
Common errors
- 1 Too little cephalad angle foreshortens the sacrum and superimposes the pubis on the foramina; 15° may need 20°.
- 2 Patient rotation obscures symmetry of sacral ala.
- 3 Pubic symphysis superimposed over sacral base.
- 4 Excessive kV adds more energetic scatter that can reach the IR.
Clinical indications
- Suspected sacral fracture from trauma.
- Evaluation of sacroiliitis or SI joint pathology.