Lateral L5/S1 spot X-ray positioning (lumbosacral junction)
One of the routine L-spine views. The patient lies on their side, the beam passes from side to side, straight on to the receptor, and the centring point is 3 cm below the iliac crest, on the mid-coronal line.
By the ScanFlo Editorial Team · Updated
In this section · X-ray positioning
- X-ray positioning guides
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- 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
Still being checked: a radiographer is reviewing this guide. If your department does it differently, follow your department.
Patient position
Lateral recumbent. Lay the patient on the left side, knees bent, a pad under the waist. Not for children.
Centring & collimation
Central ray
lateralCentre: 3 cm below the iliac crest, on the mid-coronal line.
Beam: lateral, perpendicular to the receptor.
Collimate: L4 to the upper sacrum
Distance, receptor & grid
| Setting | Value | Notes |
|---|---|---|
| SID | 100 cm | |
| Receptor | 24 × 30 cm | portrait |
| Placement | Table bucky | Grid in |
| Angulation | None | Perpendicular to the receptor |
Marker & breathing
- Marker: R or L marker on the side nearest the detector
- Breathing: Suspend respiration.
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 | 90 | 40 | 16 |
| S | 90 | 63 | 25 |
| M | 90 | 80 | 32 |
| L | 90 | 125 | 50 |
These start from a classic film-screen chart (90 kV, 125 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 L5/S1 disc space is open.
- Posterior margins superimposed.
- 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 |
|---|---|---|
| L5/S1 closed | Spine sagging | Pad the waist so the spine is horizontal. |
Common questions
- Where do you centre for a lumbosacral junction lateral X-ray?
- The centring point is 3 cm below the iliac crest, on the mid-coronal line.
- What distance and beam angle does a lumbosacral junction lateral need?
- 100 cm from the tube to the receptor. The beam passes from side to side, straight on to the receptor.
- What size receptor, and do you need a grid?
- Use a 24 × 30 cm receptor, portrait, in the table bucky. A grid is needed.
- What should the patient do with their breathing?
- Ask them to stop breathing for the moment of the exposure.
- What goes wrong most often on a lumbosacral junction lateral?
- L5/S1 closed. Usual cause: spine sagging. The fix: pad the waist so the spine is horizontal.
Try it in ScanFlo: the ScanFlo XR X-ray Room, coming soon, lets you set up a lumbosacral junction lateral in a 3D room and score your positioning. See ScanFlo XR →
Written by the ScanFlo team from everyday radiographic practice.