L5-S1 Lumbosacral Junction is a radiography positioning projection of the Spine. Lateral recumbent, CR to L5-S1 junction. Flexed knees. The centering point is located l5-s1 intervertebral space, ~1.5 inches inferior to the iliac crest and ~2 inches posterior to the asis.. The central ray is perpendicular if pelvis is supported, otherwise 5° caudad (male) or 8° caudad (female) to open l5-s1 disc space.. Image-quality criteria include l5 vertebra clearly visualized., s1 base shown below l5.. Standard exposure ranges from 90 to 100 kVp, 50 to 100 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- L5 vertebral body.
- S1 vertebral body base.
- L5-S1 intervertebral disc space.
- L5 spinous process.
- Lumbosacral alignment.
Patient preparation
- Verify patient identity using two identifiers.
- Remove jewelry, belt, and radiopaque objects.
- Lateral recumbent position, true lateral alignment.
- Hips and shoulders perpendicular to IR.
- Knees flexed for patient comfort and stability.
Position & centering point
L5-S1 intervertebral space, ~1.5 inches inferior to the iliac crest and ~2 inches posterior to the ASIS.
Central ray
Perpendicular if pelvis is supported, otherwise 5° caudad (male) or 8° caudad (female) to open L5-S1 disc space.
Exposure / technique
- kVp
- 90–100
- mAs
- 50–100
- SID
- 40" (102 cm)
- Notes
- Minor angle opens L5-S1 disc space.
Image-quality criteria
- L5 vertebra clearly visualized.
- S1 base shown below L5.
- L5-S1 intervertebral disc space open.
- Spinous processes aligned without rotation.
- Lumbosacral junction in true lateral profile.
Common errors / ARRT traps
- 1 Rotation shifts vertebral bodies off alignment.
- 2 Insufficient angle closes L5-S1 disc space.
- 3 CR not centered to L5-S1 misses junction completely.
- 4 Extended legs increase lordosis and close space.
Clinical indications
- Focused evaluation of L5-S1 disc disease.
- Assessment of lumbosacral instability or spondylolisthesis.
- Evaluation of L5-S1 strain or arthropathy.
- Post-operative L5-S1 fusion follow-up.
Aligned to the 2025 ARRT Content Specifications.