AP Oblique Lumbar Zygapophyseal Joints is a radiography positioning projection of the Spine. 45° oblique, RPO or LPO, CR to L3. Facet joints. The centering point is located l3 vertebra, with 45-degree obliquity.. The central ray is perpendicular to ir at l3.. Image-quality criteria include zygapophyseal joints clearly opened on one side., l1-l5 vertebrae visualized in oblique alignment.. Standard exposure ranges from 80 to 90 kVp, 30 to 50 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Lumbar zygapophyseal joints on oblique side.
- Lumbar pars interarticularis (bridge of Scotty dog).
- Intervertebral foramina on side of interest.
- Laminar structures in profile.
- Lumbar articular processes.
Patient preparation
- Verify patient identity using two identifiers.
- Remove jewelry, belt, and radiopaque objects.
- Supine, rotated 45 degrees (RPO or LPO position).
- Support body with positioning wedge if needed.
- Keep hips and shoulders at 45-degree angle.
Position & centering point
L3 vertebra, with 45-degree obliquity.
Central ray
Perpendicular to IR at L3.
Exposure / technique
- kVp
- 80–90
- mAs
- 30–50
- SID
- 40" (102 cm)
- Notes
- Grid required. Use RPO or LPO based on side of interest.
Image-quality criteria
- Zygapophyseal joints clearly opened on one side.
- L1-L5 vertebrae visualized in oblique alignment.
- Classic "Scotty dog" appearance on side of interest.
- No rotation beyond 45 degrees.
- Lamina of side of interest in profile.
Common errors / ARRT traps
- 1 Insufficient obliquity (less than 45°) closes facet joints.
- 2 Excessive obliquity (more than 45°) distorts vertebrae.
- 3 CR off L3 misses optimal facet joint demonstration.
- 4 Rotation not maintained during exposure.
Clinical indications
- Evaluation of zygapophyseal joint degenerative disease.
- Assessment of facet syndrome or joint hypertrophy.
- Evaluation of intervertebral foraminal narrowing.
- Suspected facet-mediated low back pain localization.
Aligned to the 2025 ARRT Content Specifications.