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

The sacroiliac (SI) joint X-ray series may be requested for patients with low back pain, suspected sacroiliitis, trauma, or ankylosing spondylitis. For radiologic technology students preparing for the ARRT registry, SI joint positioning is testable content, especially the oblique-to-side relationship; however, no fixed number of questions on this specific topic is guaranteed.

Unlike the routine pelvis or sacrum exam, an SI joint series uses specific bilateral and oblique projections selected according to the department's protocol. Clark's 12th edition prefers a PA bilateral projection, while US/ARRT-oriented teaching commonly uses an AP axial projection; these must not be blended into one universal technique. This guide separates those protocols and also covers RPO/LPO obliques, centering, evaluation criteria, and common errors.

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: A compound joint with an anterior synovial portion and a posterior syndesmotic/fibrous portion; functionally it is slightly movable
• Function: Transfers weight from the upper body to the pelvis and lower extremities, providing stability for the pelvic ring
• Orientation: The joint surfaces are oblique and pelvic/sacral inclination varies, so bilateral and oblique beam directions must follow the selected positioning protocol
• Key landmarks: ASIS (anterior superior iliac spine), iliac crests, symphysis pubis, and sacral foramina

Anatomy distinction: “Slightly movable” describes SI joint function, not its complete tissue classification. The anterior portion is synovial, while the strong posterior interosseous region is syndesmotic/fibrous. This differs from describing the entire SI joint as a simple synovial joint.

Projection 1: Bilateral SI Joints — Keep the Protocols Separate

The bilateral projection evaluates both SI joints simultaneously, but reputable positioning traditions use different patient positions, CR directions, angles, and entry points. Use the complete technique specified by your department rather than borrowing one element from another protocol.

Parameter	Clark's Preferred PA	Clark's AP Alternative	US/ARRT-Oriented AP Axial
Patient position	Prone (PA), legs extended	Supine (AP); flex hips and knees as Clark's directs to reduce lumbar lordosis	Supine (AP); use the leg position specified by the selected US/institutional protocol
IR size	24 × 30 cm (10 × 12 in), portrait	24 × 30 cm, portrait	24 × 30 cm, portrait
SID	100 cm (40 in)	100 cm	100 cm
CR angle	5–15° caudal	5–15° cranial	30° cephalad for males; 35° cephalad for females
CR centering	Midline, midway between the ASIS level and superior border of the symphysis pubis	Midline, midway between the ASIS level and superior border of the symphysis pubis	Midline, 5 cm (2 in) below the ASIS
Respiration	Suspended	Suspended	Suspended
Bucky	Yes	Yes	Yes

Patient Setup

1. First identify whether the order and department protocol call for Clark's preferred PA, Clark's AP alternative, or the US/ARRT-oriented AP axial method.
2. For Clark's PA, position the patient prone with legs extended. For Clark's AP alternative, position supine and flex the hips and knees as directed to reduce lumbar lordosis. For a US AP axial method, follow that protocol's specified leg position rather than mixing “legs extended” and “knees flexed” instructions.
3. Ensure the midsagittal plane (MSP) is perpendicular to the IR and centered to the table to avoid asymmetric SI joints.
4. Center the IR and CR together using the landmark specified by the chosen protocol.

Central Ray

• Clark's preferred PA: 5–15° caudal at midline, midway between the ASIS level and the superior symphyseal border
• Clark's AP alternative: 5–15° cranial at the same midpoint landmark
• US/ARRT-oriented AP axial: 30° cephalad for males or 35° cephalad for females, entering at midline 5 cm (2 in) below the ASIS
• Landmark caution: Clark's midpoint between the ASIS level and superior symphyseal border is not equivalent to a fixed 2.5 cm below the ASIS
• Angle selection: The angle reflects pelvic/sacral inclination and the named protocol. Do not increase it simply because a patient is larger or the joints seem deep.

Evaluation Criteria

• Symmetry: Both SI joints should be equally visible and symmetric
• Joint spaces: Both SI joints should be demonstrated according to the selected PA or AP protocol
• 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. Keep the side rule projection-specific: posterior obliques demonstrate the elevated side farthest from the IR; anterior obliques demonstrate the dependent side closest to the IR.

Parameter	RPO (Left SI Joint)	LPO (Right SI Joint)
Patient position	Supine, elevate left side 15–25° per Clark's	Supine, elevate right side 15–25° per Clark's
IR size	10 × 12 in, portrait	10 × 12 in, portrait
SID	40 in	40 in
CR angle	Perpendicular (0°)	Perpendicular (0°)
CR centering	2.5 cm (1 in) medial to elevated left ASIS	2.5 cm (1 in) medial to elevated right ASIS

How Much Obliquity?

This is protocol-dependent. Clark's 12th edition specifies 15–25° obliquity from the AP position. US-oriented teaching sources may specify 25–30°. Follow the selected departmental protocol and use image evaluation to determine whether the joint is adequately demonstrated.

Oblique Central Ray Variants

For Clark's standard posterior oblique, use a perpendicular CR 2.5 cm medial to the elevated ASIS. Do not merge that instruction with Clark's optional inferior-joint variant: when the inferior portion needs emphasis, use a 15° cranial CR, 2.5 cm medial and 5 cm inferior to the elevated ASIS.

Positioning Errors: Quick Reference

Error	Appearance	Fix
Too little obliquity	SI articulation remains superimposed	Reposition within the selected range: Clark's 15–25° or a specified US 25–30° protocol
Too much obliquity	Joint is distorted with excessive overlap	Reduce rotation within the selected protocol's range
Wrong bilateral CR protocol	Joint relationships are not demonstrated as intended	Confirm PA versus AP, then use that protocol's angle, direction, and centering landmark
Mixed centering landmarks	Anatomy may be clipped or projected at the wrong level	Use Clark's midpoint landmark or the US AP axial 5-cm-below-ASIS landmark—not a hybrid
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 — radiographs assess structural changes such as erosions, sclerosis, joint-space alteration, and ankylosis; MRI is more sensitive for early active inflammation
• Ankylosing spondylitis / axial spondyloarthritis — radiography can show established structural SI joint change, while MRI can detect active sacroiliitis earlier
• 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	Compound anterior synovial and posterior syndesmotic/fibrous joint; functionally slightly movable
Clark's preferred bilateral	PA, 5–15° caudal; midline at the midpoint between ASIS level and superior symphyseal border
Clark's AP alternative	AP, 5–15° cranial; the same midpoint landmark
US/ARRT-oriented AP axial	30° cephalad male / 35° cephalad female; midline 5 cm below ASIS
Oblique projection — RPO	Shows the LEFT SI joint (side farthest from IR)
Oblique projection — LPO	Shows the RIGHT SI joint
Oblique side rule	Posterior: elevated/farthest side; anterior: dependent/closest side
Obliquity	Clark's 15–25°; US-oriented sources may specify 25–30°
Standard oblique CR	Perpendicular, 2.5 cm medial to elevated ASIS
Inferior-joint variant	15° cranial, 2.5 cm medial and 5 cm inferior to elevated ASIS
Respiration	Suspended for all projections
Common error	Mixing the angle or centering landmark from one protocol with another
Common pathology	Sacroiliitis, ankylosing spondylitis, DJD

Clinical Pearls: Tips from the Techs

• Hard-to-position patients: A radiolucent wedge can support the elevated side at the obliquity required by the selected protocol—15–25° in Clark's or 25–30° in some US-oriented protocols—without requiring the patient to hold the position.
• 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.
• Bilateral comparison: When obliques are included by the order and local protocol, RPO + LPO allow side-to-side structural comparison. MRI, however, is more sensitive when early active inflammatory sacroiliitis is suspected.
• Do not size-adjust by habit: Choose the bilateral CR from pelvic/sacral inclination and the named protocol. The US-oriented 30° male/35° female method is not an automatic escalation for obesity or “deep” joints.

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 anatomy and keeping each positioning protocol internally consistent. The three keys to remember:

1. Bilateral protocol — Clark's prefers PA 5–15° caudal and offers AP 5–15° cranial at its midpoint landmark; the separate US AP axial method uses 30° male/35° female at 5 cm below ASIS
2. Obliques — RPO = left and LPO = right (elevated/farthest side); Clark's uses 15–25°, while some US protocols use 25–30°
3. Imaging role — radiographs evaluate structural change; MRI is more sensitive for early active sacroiliitis

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.