Etymology

• Derived from the resemblance of the radiologic appearance to a signet ring, an ornamental ring with a circular bezel often engraved with a seal.

AKA

• None.

What is it?

• The signet ring sign is a radiologic finding characterized by a dilated bronchus (the “ring”) with an adjacent smaller pulmonary artery (the “signet stone”), creating a distinctive signet ring appearance.
• It is a hallmark sign of bronchiectasis.

Characterized by

• Bronchial dilation with a diameter larger than the accompanying pulmonary artery.
• A clear contrast between the larger bronchial lumen and the smaller vessel.
• Visualized most effectively on CT.

Anatomically affecting

• Bronchi and their adjacent pulmonary arteries within the lung parenchyma.

Causes include

• Most Common Causes:
  • Post-infectious bronchiectasis (e.g., tuberculosis, bacterial pneumonia).
• Other Causes include:
  • Infection:
    • Mycobacterium avium complex (MAC).
    • Cystic fibrosis-related infections (e.g., Pseudomonas aeruginosa).
    • Allergic bronchopulmonary aspergillosis (ABPA).
  • Inflammation:
    • Chronic obstructive pulmonary disease (COPD).
    • Rheumatoid arthritis or other autoimmune diseases.
  • Neoplasm:
    • Post-obstructive bronchiectasis caused by airway tumors.
  • Mechanical:
    • Airway obstruction due to foreign bodies or strictures.
  • Inherited and Congenital:
    • Cystic fibrosis.
    • Primary ciliary dyskinesia (Kartagener syndrome).
    • Williams-Campbell syndrome.
  • Idiopathic:
    • Idiopathic bronchiectasis.

Pathophysiology

• Bronchiectasis occurs due to chronic inflammation and structural damage to the bronchial wall, leading to irreversible bronchial dilation.
• The signet ring sign represents:
  • Bronchial dilatation beyond the normal bronchial-to-arterial ratio (>1.5:1).
  • Compression or underdevelopment of adjacent pulmonary arteries, creating a size mismatch.

Histopathology

• Destruction of bronchial elastic tissue and smooth muscle.
• Chronic inflammation, fibrosis, and mucus plugging within the bronchi.

Imaging

Applied Anatomy

• Parts: Involves the bronchi and adjacent pulmonary arteries.
• Size: Bronchial diameter exceeds the accompanying artery, typically >1.5 times larger.
• Shape: Circular bronchial lumen with smaller, adjacent pulmonary artery resembling a ring.
• Position: In any lobe but most commonly affects the lower lobes.
• Character: Thickened bronchial walls with variable mucus plugging.
• Time: Chronic finding.

CXR

• May show nonspecific signs of bronchiectasis, such as:
  • Tram-track opacities (parallel bronchial walls in longitudinal sections).
  • Ring shadows in cross-sectional views.
  • Atelectasis or consolidation in affected regions.
• Less sensitive compared to CT for detecting the signet ring sign.

CT

• Key Modality:
  • Dilated bronchi appear as circular or oval structures.
  • The adjacent pulmonary artery is smaller, creating the “signet ring” appearance.
  • Bronchial wall thickening may be present, reflecting chronic inflammation.
  • Distribution:
    • Commonly affects the lower lobes (e.g., post-infectious bronchiectasis).
    • Cystic fibrosis may show upper lobe predominance.
  • Associated findings:
    • Mucus plugging (tree-in-bud appearance).
    • Mosaic perfusion indicating air trapping.
    • Atelectasis or consolidation.
    • Bronchial wall calcification in chronic cases.

MRI

• Limited utility for this sign but may demonstrate:
  • Dilated bronchi with increased signal intensity on T2-weighted imaging.

PET-CT

• May show increased metabolic activity in regions with active inflammation or infection (e.g., ABPA, MAC infection).
• Helps differentiate infectious or inflammatory processes from malignancy.

Other

• Ultrasound: Rarely used; may identify mucus-filled bronchi as echogenic structures.

Differential Diagnosis

• Normal broncho-arterial ratio: Mild variations in size can mimic a signet ring but do not exceed the 1.5:1 ratio.
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Recommendations

• Further Imaging:
  • Chest CT to confirm bronchiectasis and assess the extent of involvement.
  • High-resolution CT (HRCT) for detailed evaluation of airway abnormalities.
• Laboratory Correlation:
  • Sputum cultures for bacteria, fungi, and mycobacteria.
  • Sweat chloride testing for cystic fibrosis.
  • Serum IgE levels for ABPA.
• Functional Assessment:
  • Pulmonary function tests (PFTs) to evaluate airflow obstruction.
• Bronchoscopy: Considered in cases of suspected airway obstruction or atypical infections.

Key Points and Pearls

• The signet ring sign is a classic radiologic marker for bronchiectasis.
• It reflects irreversible bronchial dilation with adjacent pulmonary artery compression.
• CT is the gold standard for detection and characterization of the sign.
• Cystic fibrosis and post-infectious bronchiectasis are the most common causes.
• Early identification enables appropriate management to prevent complications like recurrent infections.

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This radiologic sign highlights the interplay between structure and function, emphasizing how chronic processes can create disproportionate and recognizable patterns.

 

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Parallels with Human Endeavors

The signet ring sign reflects the concept of imbalance in size and structure, which has parallels in human designs and nature:

• Art: Exaggerated proportions in sculptures and paintings, such as oversized hands or feet in symbolic works.

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• Sculpture

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• Jewelry: Traditional signet rings, where a smaller gemstone or engraving contrasts with the circular band.

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• Architecture: Misaligned columns or structures, creating a visual disproportion.

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• Nature: Asymmetry in growth patterns, such as enlarged plant stems or tree knots due to environmental stress.
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