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Scoliosis Series X-Ray Positioning: PA, Lateral, Bending Views, and Cobb Angle Guide

The scoliosis series is one of the most important radiographic examinations you'll perform as a rad tech — and one of the most commonly tested on the ARRT exam. It requires precise positioning across multiple projections, an understanding of curve mechanics, and the ability to produce images that allow accurate Cobb angle measurement for treatment decisions.

Whether you're a student encountering your first scoliosis patient or a new grad refreshing your technique, this guide covers every projection in the standard series, the positioning pitfalls that ruin measurements, and the Clark's-specific details that set your images apart.

What Is a Scoliosis Series?

A scoliosis series is a set of radiographic projections used to evaluate lateral spinal curvature. The standard protocol typically includes:

1. PA (posteroanterior) upright — the primary projection for Cobb angle measurement
2. Lateral upright — assesses sagittal alignment (kyphosis/lordosis)
3. PA upright with lateral bending — evaluates curve flexibility
4. PA Ferguson (hip/foot elevation) — differentiates structural from compensatory curves

The PA projection is preferred over AP whenever possible. This is a critical ARRT exam point: PA reduces radiation dose to anterior radiosensitive organs — particularly breast tissue in adolescent females, who make up the majority of scoliosis patients.

PA Upright — The Primary Projection

The PA upright is the workhorse of the scoliosis series. It provides the coronal plane view used for Cobb angle measurement and curve classification.

Positioning Parameters

Parameter	Specification
Patient position	Upright, facing the IR (PA)
Arms	Relaxed at sides, away from the spine
Weight bearing	Equal on both feet — level pelvis is essential
IR size	35 × 91 cm (14 × 36 inch) full-spine IR, or stitched DR
SID	72 inches (183 cm) preferred; 60 inches (152 cm) minimum
Collimation	Base of skull through ≥1 inch (2.5 cm) below iliac crests
CR	Perpendicular to IR, centered to MSP at mid-thoracolumbar region
Respiration	Suspended expiration
Grid	Yes — required for adequate penetration across variable thickness

Key Positioning Details

• Center the MSP to the IR — this is the single most important alignment step. A rotated patient produces misleading Cobb angle measurements.
• Include the entire curve — collimate superiorly to the base of skull and inferiorly to at least 1 inch below the iliac crests. Missing anatomy means missing the full curve extent.
• Level the pelvis — uneven weight bearing creates artificial pelvic tilt that can exaggerate or mask a curve.
• Use a compensating filter if available — the shoulder and pelvic regions have vastly different tissue thickness. A wedge filter balances density across the long field.
• Arms at sides — position the arms away from the spine so the humeri and scapulae don't obscure thoracic vertebrae. Have the patient place hands on hips with elbows forward if needed.

Evaluation Criteria

• Coverage: Entire spine from skull base through iliac crests centered on image; hardware included if present
• Rotation: Spinous processes midline; iliac crests and SI joints symmetric; clavicles symmetric on thoracic portion
• Motion: Clear endplates and pedicles; no breathing or sway blur
• Technique: Uniform brightness via filter/stitching; no clipped margins; shields not overlapping anatomy of interest

Lateral Upright — Sagittal Alignment

The lateral projection evaluates global sagittal alignment, including thoracic kyphosis, lumbar lordosis, and any spondylolisthesis. It's also essential for postoperative sagittal balance assessment.

Parameter	Specification
Patient position	Upright, true lateral
Weight bearing	Equal on both feet
Arms	Extended forward or grasping support bar to clear spine
Head	Neutral position, chin slightly elevated for C-spine visualization
SID	72 inches (183 cm) preferred; 60 inches (152 cm) minimum
CR	Perpendicular to mid-coronal plane at T12–L1 level
Respiration	Suspended expiration
Grid	Yes

Key Positioning Details

• True lateral stance — the posterior vertebral body borders should form a single line without double contour. If you see double borders, the patient is rotated.
• Neutral pelvis — no anterior or posterior tilt. The ASIS should be aligned vertically.
• Arms out of the way — have the patient extend arms forward or hold a support bar. The humeri and soft tissue of the upper arm can obscure the upper thoracic spine on lateral views.
• Compensating filter — use if available to balance the density difference between the dense shoulder region and the thinner pelvic region.

Evaluation Criteria

• Coverage: Entire thoracic and lumbar spine from skull base through sacrum
• Rotation: Posterior vertebral borders form a single line; ribs nearly superimposed; pelvis not rotated
• Motion: Vertebral margins sharp and distinct; no breathing blur
• Technique: Adequate penetration through shoulders and pelvis; disk spaces visible without burnout

PA Upright with Lateral Bending — Curve Flexibility

Bending views are essential for treatment planning. They distinguish structural (fixed) curves from flexible (compensatory) curves — a distinction that directly impacts bracing vs. surgical decision-making.

Parameter	Specification
Patient position	Upright PA — perform both right-bend and left-bend
Bending technique	Laterally bend from the hips without rotating the trunk or lifting heels
Arms	Position to avoid obscuring ribs and lumbar spine
SID	72 inches (183 cm) preferred
CR	Perpendicular to MSP at mid-thoracolumbar region
Collimation	Same as PA upright — skull base through iliac crests

Clinical Significance

• Structural curve: Does NOT correct significantly with bending (>25° residual Cobb angle on the bend film). This curve is fixed and likely requires surgical intervention.
• Flexible curve: Corrects substantially with bending (<25° residual). This curve may respond to bracing.
• Preoperative planning: Surgeons use bending views to determine which curves are primary (need correction) and which are compensatory (will self-correct once the primary curve is addressed).

PA Ferguson — Hip/Foot Elevation Test

The Ferguson view is a supplemental projection that uses hip or foot elevation to differentiate primary from compensatory curves based on how the curve responds to pelvic obliquity.

Positioning

1. First image: Neutral upright PA (same as routine)
2. Second image: PA with hip/foot elevated 3–4 inches (8–10 cm) on the convex side of the primary curve
3. Patient must be unsupported beyond the block — effort is required to maintain position
4. Identical centering, collimation, and technique for both images

Clinical Interpretation

• Primary (structural) curve: Minimal or no change with elevation — the curve is fixed regardless of pelvic position
• Compensatory curve: Significant correction with elevation — the curve adapts to pelvic obliquity, confirming it's secondary to the primary curve

Cobb Angle Measurement

The Cobb angle is the gold-standard measurement for quantifying scoliosis severity. Understanding how it's measured — and what the numbers mean — is essential for both clinical practice and the ARRT exam.

How to Measure

1. Identify the most tilted vertebra above the curve apex (the vertebra with the greatest endplate tilt toward the concavity)
2. Identify the most tilted vertebra below the curve apex
3. Draw a line along the superior endplate of the upper vertebra
4. Draw a line along the inferior endplate of the lower vertebra
5. Extend these lines until they intersect (or draw perpendiculars from each line if they don't intersect within the image)
6. The angle formed at the intersection is the Cobb angle

Clinical Thresholds

Cobb Angle	Classification	Clinical Action
<10°	Normal variation	No treatment; monitor
10–25°	Mild scoliosis	Observation; repeat imaging in 6–12 months
25–40°	Moderate scoliosis	Bracing consideration (skeletally immature patients)
40–50°	Severe scoliosis	Surgical consultation recommended
>50°	Very severe	Surgery almost always indicated

Measurement Pitfalls

• Vertebral rotation can make endplates harder to identify — use the pedicle shadows as landmarks
• Inter-observer variability is real — different radiologists may measure slightly different angles. This is why identical positioning on follow-up studies is critical for accurate comparison.
• Upper thoracic curves can be difficult to measure on standard-length IR — full-spine stitching is preferred

Curve Classification Systems

King Classification (Historical)

The King classification divided thoracic curves into 5 types (I–V) based on curve pattern and flexibility. It was widely used but has been largely replaced by the Lenke system.

Lenke Classification (Current Standard)

The Lenke classification is the modern standard, categorizing curves by:

1. Curve type (1–6) — based on which regions are involved (proximal thoracic, main thoracic, thoracolumbar/lumbar)
2. Lumbar modifier (A, B, C) — based on the relationship of the lumbar curve to the center sacral vertical line
3. Sagittal thoracic modifier (–, N, +) — based on the T5–T12 kyphosis angle

This system provides more detailed information for surgical planning than the King classification.

Pediatric Positioning Considerations

Scoliosis most commonly affects adolescents, so you'll frequently position pediatric patients for this examination. Key modifications:

• Smaller IR — use the largest IR that fits the patient; full-spine IR (35 × 91 cm) is ideal but may be too large for small children. Stitched DR images are an excellent alternative.
• Child life specialist — for younger or anxious patients, involve child life to help with cooperation and positioning instructions.
• Modified technique — use lower kVp and mAs appropriate for pediatric body habitus. Follow ALARA principles.
• Shielding — breast and gonadal shielding when feasible without obscuring the spine. Note: some facilities have moved away from shielding per Image Gently recommendations — follow your department protocol.
• PA projection especially important — the PA preference is even more critical in pediatric patients due to developing breast tissue and greater radiosensitivity.
• Reproducibility — ensure identical positioning on follow-up studies. Curve progression is tracked over months to years, and inconsistent positioning introduces measurement error.

Common Positioning Errors and How to Avoid Them

Error	Effect on Image	How to Avoid
Patient rotation	Misleading Cobb angle; asymmetric iliac crests	Center MSP precisely; check spinous process alignment before exposure
Uneven weight bearing	Artificial pelvic tilt; curve exaggeration	Verify equal weight on both feet; check ASIS level
Insufficient coverage	Missing curve extent; incomplete Cobb angle measurement	Collimate skull base through ≥1" below iliac crests
Arms obscuring spine	Thoracic vertebrae hidden behind humeri	Position arms at sides (PA) or forward (lateral)
Motion blur	Unreadable endplates; inaccurate measurement	Instruct patient to hold still; use shortest possible exposure time
No compensating filter	Overexposed shoulders or underexposed pelvis	Use wedge filter or ensure stitching algorithm handles density variation
Inconsistent follow-up positioning	False impression of curve progression or correction	Document exact positioning parameters; use same SID, centering, and stance

Clark's Pocket Handbook References

Clark's Pocket Handbook for Radiographers provides specific positioning guidance for the scoliosis series:

• PA scoliosis (Clark's): Upright PA projection with full-spine IR. Center MSP to IR. Include skull base through iliac crests. Use compensating filter for uniform density.
• Lateral scoliosis (Clark's): True lateral upright. Arms forward to clear spine. CR perpendicular to mid-coronal plane.
• Note on PA vs AP: Clark's specifies PA as the preferred projection for dose reduction to anterior organs — consistent with current ALARA and Image Gently principles.

Where Clark's differs from other references, default to Clark's. For example, some texts recommend AP for scoliosis because it's easier to position; Clark's maintains PA preference based on dose considerations.

Related Articles

• Thoracic and Lumbar Spine X-Ray Positioning — AP, lateral, and oblique projections for the T-spine and L-spine
• Cervical Spine X-Ray Positioning — 5-view C-spine series with Clark's centering points
• Pediatric Radiography Techniques — Modified positioning and immobilization for children
• Radiation Safety for Radiologic Technologists — ALARA principles and dose reduction strategies
• X-Ray Physics Made Simple — SID, magnification, and exposure factor fundamentals