Signet Ring Sign on CT Chest
The signet ring sign on CT chest indicates bronchiectasis, characterized by abnormal bronchial dilation where the internal diameter of a bronchus appears larger than its accompanying pulmonary artery. 1
Radiologic Definition and Appearance
The signet ring sign represents the key diagnostic feature of bronchiectasis on high-resolution CT (HRCT) scanning, where the dilated bronchus (appearing as the "ring") sits adjacent to its accompanying pulmonary artery (appearing as the "signet" or stone). 1 This creates a characteristic appearance resembling a signet ring when viewed in cross-section. 2, 3
The bronchus-to-artery ratio exceeds 1:1 when this sign is present, confirming pathologic airway dilation. 1
Clinical Significance
Primary Diagnosis
- HRCT scanning has sensitivity and specificity exceeding 90% for diagnosing bronchiectasis, making it the diagnostic procedure of choice when this condition is suspected. 1
- The signet ring sign is more reliable than conventional chest radiographs, which may appear normal in early bronchiectasis or show only nonspecific findings. 1
Associated CT Features
Additional HRCT findings that accompany the signet ring sign in bronchiectasis include: 1
- Failure of larger airways to taper normally as they progress toward the lung periphery
- Air-fluid levels within dilated airways
- Identification of airways in the extreme lung periphery (where they should not normally be visible)
- Bronchial wall thickening
- Mucoid impaction
- Focal air-trapping
Differential Considerations
Alternative Context: Pulmonary Hydatid Disease
In rare circumstances, a "signet ring sign" has been described in pulmonary hydatid cysts, where it represents a bleb of air in the wall of an as-yet unruptured cyst. 4 However, this represents a completely different pathologic process and imaging context than the classic bronchiectasis finding. The hydatid cyst variant is uncommon and should not be confused with the standard bronchiectasis definition. 4
Clinical Action Required
When the signet ring sign is identified on CT chest: 1
- Confirm the diagnosis of bronchiectasis and initiate systematic evaluation for underlying causes
- Investigate for post-infectious etiologies (childhood pneumonia, pertussis, measles, tuberculosis)
- Screen for cystic fibrosis, especially if mucoid Pseudomonas is isolated
- Evaluate for allergic bronchopulmonary aspergillosis if Aspergillus is present
- Consider immunodeficiency disorders (common variable immunodeficiency)
- Assess for Mycobacterium avium complex infection
- Rule out traction bronchiectasis from fibrosing lung diseases
Sputum Culture Interpretation
Common organisms isolated include Haemophilus influenzae, Staphylococcus aureus, Streptococcus pneumoniae, and Pseudomonas aeruginosa, though these are not specific for bronchiectasis as they may also colonize airways in chronic bronchitis. 1
Common Pitfalls
- Do not rely on physical examination findings alone: crackles on auscultation do not correlate reliably with CT-diagnosed bronchiectasis. 1
- Do not assume all chronic cough with sputum production is bronchiectasis: in one prospective study, bronchiectasis accounted for only 4% of cases with >30 mL daily sputum production, with upper airway cough syndrome (40%), asthma (24%), and GERD (15%) being more common. 1
- Recognize that 47% of bronchiectasis cases have identifiable underlying causes, with 15% having diagnoses that significantly impact treatment decisions. 1