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, about 2 inches medial to the elevated ASIS, 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. A sample technique is 90 kVp with DR 18 or CR 36 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Lumbar zygapophyseal joints on the down side, nearest the IR.
- Lumbar pars interarticularis, the neck of the Scotty dog.
- Superior and inferior articular processes forming the joint.
- Pedicle, the eye of the Scotty dog.
- 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 the shoulders and pelvis with radiolucent sponges.
- Keep equal rotation of the shoulders and pelvis.
Position & centering point
L3 vertebra, about 2 inches medial to the elevated ASIS, with 45-degree obliquity.
Central ray
Perpendicular to IR at L3.
Exposure / technique
- kVp
- 90
- mAs
- DR 18 / CR 36
- SID
- 40" (102 cm)
- Notes
- Grid required. Use RPO or LPO based on side of interest. Sample technique: DR 18 / CR 36 mAs at 90 kVp, 40 in SID, for a 23 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
- Zygapophyseal joints clearly opened on one side.
- L1-L5 vertebrae visualized in oblique alignment.
- Classic "Scotty dog" on the down side, nearest the IR: RPO shows the right, LPO the left.
- 45 degrees suits the general lumbar region; Bontrager uses 50 for L1-L2 and 30 for L5-S1, Merrill's up to 60 for L5-S1.
- Pedicle between the midline and the lateral vertebral border.
Common errors
- 1 Under-rotation: pedicle lies laterally on the vertebral border, with more lamina showing.
- 2 Over-rotation: pedicle lies closer to the midline, with less of it showing.
- 3 CR off L3 misses optimal facet joint demonstration.
- 4 No radiolucent support under the shoulders and pelvis, so the position is not maintained.
Clinical indications
- Evaluation of a pars interarticularis defect (spondylolysis).