L5-S1 Lumbosacral Junction is a radiography positioning projection of the Spine. Lateral recumbent, knees flexed. CR 1.5 in below the iliac crest and 2 in posterior to the ASIS; perpendicular with waist support, otherwise 5 to 8 degrees caudad. 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 with sufficient waist support, otherwise (Bontrager) 5-8° caudad, parallel to the interiliac line. Merrill's gives 5° caudad for men and 8° for women, and shows a cephalad angle in a large waist. Image-quality criteria include L5 vertebra clearly visualized, S1 base shown below L5. A sample technique is 96 kVp with DR 45 or CR 100 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- L5 vertebral body.
- S1 vertebral body base.
- L5-S1 intervertebral disc space.
- First and second sacral segments.
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 with sufficient waist support, otherwise (Bontrager) 5-8° caudad, parallel to the interiliac line. Merrill's gives 5° caudad for men and 8° for women, and shows a cephalad angle in a large waist.
Exposure / technique
- kVp
- 96
- mAs
- DR 45 / CR 100
- SID
- 40" (102 cm)
- Notes
- An angle is needed only when the waist support leaves the spine sagging, to stay parallel to the interiliac line. Bontrager prints 90 +/- 5 kV for digital systems; the sample below is from Merrill's chart. Sample technique: DR 45 / CR 100 mAs at 96 kVp, 40 in SID, for a 31 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
- L5 vertebra clearly visualized.
- S1 base shown below L5.
- L5-S1 intervertebral disc space open.
- No rotation: greater sciatic notches and posterior vertebral body borders superimposed.
- Lumbosacral junction in true lateral profile.
Common errors
- 1 Rotation shifts vertebral bodies off alignment.
- 2 Spine sagging with no CR angle parallel to the interiliac line closes the L5-S1 joint space.
- 3 CR not centered to L5-S1 misses junction completely.
Clinical indications
- Other L5-S1 pathologies.
- Spondylolisthesis involving L4-L5 or L5-S1.