AXIAL AP L5-S1
Axial Projection of the Lumbosacral Joint • Evaluation of the L5-S1 Disc
Pathology Demonstrated
Primary Purpose: Specific evaluation of the lumbosacral joint and its components
Key Structures: L5-S1 intervertebral disc, lumbosacral facet joints, intervertebral space
Exposure Factors
Reduced mAs: 20 mAs (lower than standard AP lumbar) due to a more limited field
Visible Anatomical Structure
Specific Focus: Direct visualization of the L5-S1 intervertebral space without superimposition
Plate Size
Orientation: Transverse
Justification: Limited field for the specific L5-S1 joint
Note: "An 18x24cm cassette placed transversely is normally used"
Patient Positioning
Supine Position
Leg Elevator
"The legs should be supported on an elevator at knee height, so that the lumbar area is in contact with the table"
- Reduces physiological lumbar lordosis
- Allows for complete contact of the spine with the table
- Facilitates cephalic angulation of the beam
- Improves visualization of the L5-S1 space
Purpose: To flatten the lumbar curve to project an open intervertebral space
Differential Angulation by Gender
"Cephalic angulation of 30º for men and 35º for women"
MEN
Cephalic angulation
Justification: Narrower pelvis, smaller sacral angle
Lesser inclination of the sacrum
WOMEN
Cephalic angulation
Justification: Wider pelvis, larger sacral angle
Greater inclination of the sacrum
The difference is due to anatomical variations of the pelvis between genders (gynecoid vs. android pelvis)
Central Ray Point
Location: "Center at the level of the ASIS centered on the midline of the body"
Reference: Anterior Superior Iliac Spine (ASIS)
Angulation: Cephalic 30° (men) / 35° (women)
Direction: From inferior to superior, passing through the L5-S1 space
Optimal Image Characteristics
L5-S1 Space
Open and symmetrical
Joints
Facets visible
Endplates
Parallel and defined
Symmetry
Uniform intervertebral space
No Superimposition
Iliac crests not superimposed
Correct Angulation
According to the patient's gender
Common Technical Challenges
Frequent problems in Axial AP L5-S1 projection:
- Incorrect angulation by gender (not differentiating 30° vs 35°)
- Leg elevator missing or incorrectly placed
- Pelvic rotation resulting in an asymmetrical image
- Persistent lumbar lordosis that closes the L5-S1 space
- Incorrect centering (not at the ASIS and midline)
- Iliac crest superimposition over the L5-S1 space
- Breathing during exposure resulting in blurriness
- Plate too large (not using 18×24 cm transverse)
Solution: Use a leg elevator, verify lack of rotation, apply correct angulation based on gender
Special Considerations
Mobility Limitation
Patients who cannot extend legs require angulation adjustment and a possible alternative technique.
Spondylolisthesis
In grade I-II listhesis, adjust angulation to visualize the intervertebral space.
Lumbar Scoliosis
Align according to the primary curve. May require compensatory angulation.
Obesity
Increase kV/mAs. Difficulty in achieving full lumbar contact with the table.
Specific Indications
This projection is specifically indicated for:
- L5-S1 disc herniations
- Lumbosacral foraminal stenosis
- Spondylolisthesis grade I-II of L5 on S1
- L5-S1 disc degeneration
- Lumbosacral facet arthrosis
- Pre-surgical evaluation for L5-S1 fusion
Patient Instructions
"During the exposure, the patient must not breathe"
Full sequence:
1. "Lie on your back"
2. "Support your legs on the elevator"
3. "Relax your back against the table"
4. "Do not turn your hip or torso"
5. "Take a deep breath and then let all the air out"
6. "Hold your breath and do not move"
7. "Relax when I tell you"