Home› Articles› SI Joint Positioning

SI (Sacroiliac) Joint X-Ray Positioning: AP Axial, RPO, and LPO Views

The sacroiliac (SI) joint X-ray series is one of the most commonly requested exams for patients presenting with low back pain, suspected sacroiliitis, or ankylosing spondylitis. For radiologic technology students preparing for the ARRT registry, SI joint positioning is a high-yield topic — expect 2–3 questions on the oblique-to-side relationship alone.

Unlike the routine pelvis or sacrum exam, the SI joint series uses specific oblique projections and a cephalad CR angle to open up the joint space and evaluate each side separately. This guide covers the two standard views — AP axial and RPO/LPO obliques — with Clark's-sourced centering points, evaluation criteria, and the common errors that even experienced techs make.

Anatomy of the Sacroiliac Joints

Before you position, know what you are imaging. The SI joints are formed by the auricular surface of the sacrum articulating with the auricular surface of each ilium.

• Joint type: Amphiarthrodial (slightly movable) synovial joint — a favorite ARRT exam question
• Function: Transfers weight from the upper body to the pelvis and lower extremities, providing stability for the pelvic ring
• Orientation: The joint plane runs obliquely — approximately 25° from the coronal plane, which is why you need an angled approach (both cephalad and oblique) to visualize it
• Key landmarks: ASIS (anterior superior iliac spine), iliac crests, symphysis pubis, and sacral foramina

Notable Clark's distinction: Clark's Pocket Handbook specifies the SI joints as amphiarthrodial joints with an oblique orientation of approximately 25° from the coronal plane — a key difference from the sacrococcygeal joint, which is a cartilaginous symphysis.

Projection 1: AP Axial (Routine and Ferguson Method)

This is the primary projection for evaluating both SI joints simultaneously. It uses a cephalad CR angle to project through the obliquely oriented joint space.

Parameter	AP Axial (Routine)	AP Axial (Ferguson Modification)
Patient position	Supine, legs extended, knees may be flexed with support	Supine, legs extended
IR size	10 × 12 in (24 × 30 cm), portrait	10 × 12 in, portrait
SID	40 in (100 cm)	40 in
CR angle	15° cephalad	30–35° cephalad
CR centering	Midline, halfway between ASIS and symphysis pubis	Midline at the level of the ASIS
Respiration	Suspended	Suspended
Bucky	Yes	Yes

Patient Setup

1. Position the patient supine on the X-ray table with legs extended.
2. Place a small radiolucent support under the knees to reduce lumbar lordosis — this brings the sacrum more parallel to the IR.
3. Center the IR at the level of the ASIS (approximately 2 in / 5 cm below the iliac crest).
4. Ensure the midsagittal plane (MSP) is perpendicular to the IR and centered to the midline of the table.

Central Ray

• Angle: 15° cephalad (routine) or 30–35° cephalad (Ferguson method)
• Entry point: Midline, halfway between the ASIS and the symphysis pubis — approximately 2.5 cm below the level of the ASIS
• Direction: The CR passes through the sacrum between the two SI joints, projecting the joint spaces open

Evaluation Criteria

• Symmetry: Both SI joints should be equally visible and symmetric
• Joint spaces: The SI joint spaces should appear open — the cephalad angle separates the joint surfaces
• Sacral foramina: The anterior sacral foramina should be visible through the sacrum without superimposition
• No rotation: The spinous processes of the lumbar spine should align with the midline of the sacrum
• Collimation: Include the iliac crests superiorly and the pubic symphysis inferiorly

Projection 2: RPO and LPO Obliques

The oblique projections evaluate one SI joint at a time. The key concept for the ARRT exam: the side closest to the image receptor is the side demonstrated.

Parameter	RPO (Left SI Joint)	LPO (Right SI Joint)
Patient position	Supine, rotate 25° with left side elevated	Supine, rotate 25° with right side elevated
IR size	10 × 12 in, portrait	10 × 12 in, portrait
SID	40 in	40 in
CR angle	Perpendicular (0°)	Perpendicular (0°)
CR centering	1 in medial to the ASIS of the elevated side	1 in medial to the ASIS of the elevated side

Why 25°?

This is a Clark's distinction: Clark's Pocket Handbook specifies 25° obliquity from the AP position. Some textbooks recommend 25–30°, but Clark's 25° is the most consistently accurate for opening the joint space without distorting the articular surfaces.

Positioning Errors: Quick Reference

Error	Appearance	Fix
Under-rotation (<20°)	SI joint appears closed or overlapped	Increase rotation by 5–10°
Over-rotation (>35°)	Joint space distorted, sacral ala overlies ilium	Decrease rotation
Insufficient CR angle	Sacrum foreshortened, SI joints not open	Increase cephalad angle by 5°
Excessive CR angle	Pubic symphysis superimposed over sacrum	Decrease cephalad angle
Patient rotation on AP axial	Asymmetric SI joints, spinous processes off midline	Reposition using MSP and ASIS landmarks
Motion (breathing)	Blurred joint margins	Reinforce suspended respiration instruction

Clinical Indications

The SI joint series is commonly ordered for:

• Sacroiliitis — inflammation of one or both SI joints, often associated with seronegative spondyloarthropathies
• Ankylosing spondylitis — progressive fusion of the SI joints as the earliest radiographic sign
• Trauma — pelvic ring injuries involving the SI joint (Tile type B/C fractures)
• Degenerative joint disease — osteoarthritis of the SI joint
• Post-surgical evaluation — following SI joint fusion or pelvic reconstruction

ARRT Exam Prep: SI Joint Quick Review

Fact	Details
Joint type	Amphiarthrodial (slightly movable) synovial joint
AP axial CR angle	15° cephalad (routine) or 30–35° cephalad (Ferguson)
AP axial centering	Midline, halfway between ASIS and symphysis pubis
Ferguson method	30–35° cephalad, often used for larger patients
Oblique projection — RPO	Shows the LEFT SI joint (side farthest from IR)
Oblique projection — LPO	Shows the RIGHT SI joint
Clark's obliquity	25° from the AP position
CR on obliques	Perpendicular, 1 in medial to ASIS of elevated side
Respiration	Suspended for all projections
Common error	Under-rotation (<20°) — SI joint appears closed
Common pathology	Sacroiliitis, ankylosing spondylitis, DJD

Clinical Pearls: Tips from the Techs

• Hard-to-position patients: For patients with limited mobility who cannot rotate 25°, place a 30° foam wedge under the elevated hip to achieve adequate obliquity without active patient participation.
• Anterior oblique alternative: If the patient cannot tolerate supine positioning (post-surgical, severe pain), RAO/LAO projections achieve similar results with the patient prone. RAO = Right SI joint; LAO = Left SI joint.
• Always bilateral: For suspected sacroiliitis, perform RPO + LPO for comparison. Early sacroiliitis may be unilateral, and the asymptomatic side serves as the patient's own normal baseline.
• Ferguson vs routine: Use the 30–35° Ferguson angle for larger patients or when the SI joints are particularly deep. The steeper angle projects the joint space more effectively when the sacrum is steeply angled.

For related imaging of the pelvis and spine, see our guides on Pelvis X-Ray Positioning and Sacrum and Coccyx Positioning.

Test Your Knowledge — ARRT-Style Practice Questions

Summary

Mastering the SI joint X-ray series requires understanding the oblique anatomy of the joint itself and how the CR angle and patient obliquity work together to open the joint space. The three keys to remember:

1. AP Axial — 15° cephalad, centered halfway between ASIS and symphysis pubis, both joints evaluated simultaneously
2. RPO = Left, LPO = Right — 25° obliquity, CR perpendicular to the ASIS of the elevated side
3. Evaluation — Open joint space, visible sacral foramina, symmetric and without rotation

For more guidance on pelvis and spine positioning, check out our related guides: Pelvis X-Ray Positioning, Sacrum and Coccyx Positioning, and the X-Ray modality page.