13.2 Sacrum, Coccyx, Sacroiliac Joints & Bony Thorax (Ribs & Sternum)

13.2 Sacrum, Coccyx, Sacroiliac Joints & Bony Thorax (Ribs & Sternum)

Radiography of the sacrum, coccyx, sacroiliac (SI) joints, and bony thorax (sternum and ribs) demands specialized angulations, precise respiration control, and tailored contrast parameters to optimize visualization of these unique osseous structures.

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Sacrum & Coccyx Radiography

1. Sacrum Radiography

• AP Axial Sacrum:
  • CR Angle: 15° cephalad (compensates for anterior sacral curvature, aligning CR perpendicular to sacral body).
  • Centering: Midway between ASIS and pubic symphysis (approximately 2 inches superior to pubic symphysis).
  • Demonstrated Anatomy: Sacral body, anterior sacral foramina, and sacral alae free of foreshortening or superimposition of pubic bones.
• Lateral Sacrum:
  • CR Angle: Perpendicular to IR.
  • Centering: Level of ASIS, 3 to 4 inches posterior to ASIS.
  • Demonstrated Anatomy: Sacral promontory, sacral canal, posterior sacral wall, and L5–S1 articulation.

2. Coccyx Radiography

• AP Axial Coccyx:
  • CR Angle: 10° caudal (angles central ray below pubic symphysis to prevent superimposition over coccygeal segments).
  • Centering: 2 inches superior to pubic symphysis at mid-sagittal plane.
  • Demonstrated Anatomy: Coccygeal segments (1–4) free of pubic bone superimposition.
• Lateral Coccyx:
  • CR Angle: Perpendicular to IR.
  • Centering: 3 to 4 inches posterior to ASIS and 2 inches inferior.
  • Demonstrated Anatomy: Coccygeal curvature, intercoccygeal joints, and sacrococcygeal articulation.

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Sacroiliac (SI) Joints Radiography

1. AP Axial Sacroiliac Joints

• CR Angle: 30° to 35° cephalad (30° for males, 35° for females to accommodate gender differences in pelvic tilt).
• Centering: Midline of body, 2 inches inferior to ASIS (at level of pubic symphysis).
• Demonstrated Anatomy: Bilateral open SI joints and L5–S1 intervertebral joint.

2. AP Oblique SI Joints (RPO & LPO)

• Body Rotation: 25° to 30° oblique rotation from supine position.
• Centering: CR perpendicular, entering 1 inch medial to the elevated ASIS.
• Demonstrated Anatomy: The AP oblique positions (RPO and LPO) demonstrate the SI joint on the UPSIDE / ELEVATED side (farthest from the IR). E.g., LPO demonstrates the right SI joint open; RPO demonstrates the left SI joint open. (Conversely, PA obliques RAO/LAO show downside SI joints).

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Bony Thorax: Sternum Radiography

1. Sternum Anatomy & Landmarks

The sternum consists of three parts: manubrium (superior), body/gladiolus (middle), and xiphoid process (inferior). Key surface landmarks include:

• Jugular Notch (Manubrial Notch): Level of T2–T3.
• Sternal Angle (Angle of Louis): Level of T4–T5 (junction of manubrium and body).
• Xiphoid Process Tip: Level of T9–T10.

2. RAO Projection of Sternum

• Why RAO instead of LAO? The sternum lies directly anterior to the vertebral column and thoracic aorta. Performing a 15° to 20° Right Anterior Oblique (RAO) rotates the sternum into the homogenous radiolucent shadow of the heart while shifting the dense vertebral column away from the field of view. An LAO position would project the sternum over the lungs and spine, reducing contrast.
• Degree of Rotation: Thin/shallow-chested patients require greater rotation (up to 20°); deep/large-chested patients require less rotation (15°).
• Technical Factors & Breathing Technique:
  • Low kVp (60–70 kVp): Provides high subject contrast.
  • Orthostatic Breathing Technique: Low mA (25–50 mA) with long exposure time (2 to 3 seconds) while patient breathes gently. This blurs out pulmonary lung markings and rib lines while preserving crisp detail of the sternum.

3. Lateral Sternum

• SID: Extended to 72 inches (180 cm) to minimize magnification and OID blur.
• Positioning: Patient standing/erect, chest puffed out, hands clasped tightly behind back to retract shoulders posteriorly.
• CR: Perpendicular to mid-sternum (midway between jugular notch and xiphoid process).
• Demonstrated Anatomy: Sternal body, manubrium, and xiphoid process in profile without shoulder superimposition.

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Bony Thorax: Rib Radiography

1. Anatomic Classification

• True Ribs (Ribs 1–7): Attach directly to sternum via costal cartilages.
• False Ribs (Ribs 8–10): Attach indirectly to sternum via costal cartilage of rib 7.
• Floating Ribs (Ribs 11–12): No anterior attachment; end in abdominal musculature.

2. Technical & Respiration Protocol Based on Diaphragm & Pain Location

A. Ribs ABOVE Diaphragm (Ribs 1–10)

• Patient Position: Erect (gravity pulls diaphragm to lowest position).
• Respiration: Full INSPIRATION (expands lungs and depresses diaphragm below rib 10).
• Exposure Factors: Higher kVp (70–80 kVp) to penetrate dense pulmonary structures.

B. Ribs BELOW Diaphragm (Ribs 8–12)

• Patient Position: Supine (abdominal organs push diaphragm upward, elevating it above lower ribs).
• Respiration: Full EXPIRATION (further elevates diaphragm above lower rib margins).
• Exposure Factors: Lower kVp (65–75 kVp) to enhance contrast against soft tissue abdominal viscera.

C. Posterior vs. Anterior Pain Protocol

• Anterior Rib Injury: Perform PA projection with affected side against IR to minimize OID.
• Posterior Rib Injury: Perform AP projection with affected side against IR to minimize OID.

D. Axillary Rib Obliques (45° Body Rotation)

To demonstrate the axillary (lateral) portion of the ribs free of superimposition:

• AP Oblique (RPO / LPO): Demonstrates the axillary ribs on the DEPENDENT / CLOSEST side to IR (e.g., RPO shows right axillary ribs).
• PA Oblique (RAO / LAO): Demonstrates the axillary ribs on the ELEVATED / AWAY side from IR (e.g., RAO shows left axillary ribs).

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Bony Thorax Positioning Summary Table