AP sacrum and SI joints X-ray positioning
One of the routine sacrum & SI joints views. The patient lies on their back, the beam passes front to back (AP), tilted 15° towards the head, and the centring point is 3 cm above the symphysis.
By the ScanFlo Editorial Team · Updated
In this section · X-ray positioning
- X-ray positioning guides
- Skull positioning
- Face & sinuses positioning
- Soft tissue neck positioning
- C-spine positioning
- Chest positioning
- Shoulder positioning
- Humerus positioning
- Elbow positioning
- Forearm positioning
- Wrist positioning
- Hand positioning
- T-spine positioning
- Abdomen positioning
- L-spine positioning
- Sacrum & SI joints positioning
- Pelvis positioning
- Hip positioning
- Femur positioning
- Knee positioning
- Tibia & fibula positioning
- Ankle positioning
- Foot positioning
X-ray positioning/Sacrum & SI joints
Still being checked: a radiographer is reviewing this guide. If your department does it differently, follow your department.
Patient position
Supine. Supine, legs straight, mid-sagittal plane central. Not for children.
Centring & collimation
Central ray
APCentre: 3 cm above the symphysis.
Beam: AP, angled 15° cephalad.
Collimate: L5 to the coccyx, both sacroiliac joints
Distance, receptor & grid
| Setting | Value | Notes |
|---|---|---|
| SID | 100 cm | |
| Receptor | 24 × 30 cm | landscape |
| Placement | Table bucky | Grid in |
| Angulation | 15° | cephalad |
Marker & breathing
- Marker: anatomical side marker on the side being examined, clear of anatomy
- Breathing: Quiet breathing.
Exposure by patient size
Treat these as a starting point. Your detector, your generator and the patient in front of you will all move the numbers.
| Patient size | kV | mAs (chart) | mAs (DR default) |
|---|---|---|---|
| XS | 70 | 16 | 6.3 |
| S | 70 | 25 | 10 |
| M | 70 | 32 | 12.5 |
| L | 70 | 50 | 20 |
These start from a classic film-screen chart (70 kV, 63 mAs at 140 cm) and are adjusted for a digital detector at 100 cm. The DR column is what ScanFlo's console suggests; each step up or down changes the mAs by about a quarter.
Image quality checklist
- The sacrum is shown without foreshortening.
- Both sacroiliac joints are symmetric.
- The anatomical side marker is visible, correct and clear of the anatomy.
- Collimation borders are visible on all sides, no wider than the area requested.
Faults & how to fix them
| Fault | Cause | Fix |
|---|---|---|
| Sacrum foreshortened | No cephalad angle | Angle 15° cephalad. |
Common questions
- Where do you centre for a sacrum AP and sacroiliac joints X-ray?
- The centring point is 3 cm above the symphysis.
- What distance and beam angle does a sacrum AP and sacroiliac joints need?
- 100 cm from the tube to the receptor. The beam passes front to back (AP), tilted 15° towards the head.
- What size receptor, and do you need a grid?
- Use a 24 × 30 cm receptor, landscape, in the table bucky. A grid is needed.
- What should the patient do with their breathing?
- They can keep breathing gently; no breath-hold is needed.
- What goes wrong most often on a sacrum AP and sacroiliac joints?
- Sacrum foreshortened. Usual cause: no cephalad angle. The fix: angle 15° cephalad.
Try it in ScanFlo: the ScanFlo XR X-ray Room, coming soon, lets you set up a sacrum ap and sacroiliac joints in a 3D room and score your positioning. See ScanFlo XR →
Written by the ScanFlo team from everyday radiographic practice.