Bronchiectasis on CT Scans: Key Diagnostic Criteria and Imaging Features

When diagnosing bronchiectasis, high-resolution computed tomography (HRCT) has become the gold standard due to its ability to provide detailed visualization of the bronchial tree. Radiologists rely on a combination of direct and indirect imaging findings to confirm the presence of this chronic lung condition. Understanding these criteria is essential for accurate diagnosis, effective treatment planning, and long-term patient management.

Direct CT Signs of Bronchiectasis

The most definitive evidence of bronchiectasis comes from direct morphological changes visible on CT scans. These structural abnormalities are considered hallmark features and include several key signs:

1. Bronchial Dilation: The Primary Indicator

A fundamental diagnostic criterion is abnormal widening of the bronchial lumen. This occurs when the diameter of a bronchus exceeds that of the accompanying pulmonary artery—commonly referred to as the "artery-to-bronchus ratio" being greater than 1. This finding is especially reliable in peripheral lung zones and helps differentiate pathological dilation from normal anatomical variation.

2. The "Signet Ring" Sign

This classic imaging feature appears when a dilated bronchus is seen in cross-section adjacent to a smaller pulmonary artery, resembling a signet ring. It typically occurs in bronchi oriented perpendicular to the scanning plane and is one of the most recognizable indicators of bronchiectasis on axial CT images.

3. The "Tram-Track" Sign

When bronchi are visualized in longitudinal section, parallel linear opacities representing thickened, dilated bronchial walls appear side by side—resembling tram or train tracks. This "tram-track" sign is commonly observed in medium-sized airways and supports the diagnosis, particularly in cylindrical forms of bronchiectasis.

4. Visualization of Peripheral Airways

In healthy lungs, small airways beyond the sixth generation are usually not visible on CT. However, in bronchiectasis, these peripheral bronchi become clearly detectable due to persistent dilation and wall thickening—a clue that points strongly toward underlying disease.

5. Cystic or Saccular Bronchial Changes

In more advanced cases, bronchi may transform into clustered, air-filled cysts with irregular walls. These cystic spaces often contain air-fluid levels and are typically found in the lower lobes. This pattern is characteristic of severe or long-standing bronchiectasis and may be associated with recurrent infections.

Indirect Imaging Findings Supporting the Diagnosis

While not specific on their own, indirect CT signs often accompany direct findings and contribute valuable context to the overall assessment of bronchiectasis.

1. Bronchial Wall Thickening

Chronic inflammation leads to fibrosis and thickening of the bronchial walls, which appear as prominent, irregular tubular structures on imaging. This change reflects ongoing airway damage and is frequently seen alongside luminal dilation.

2. Mucus-Filled Bronchi

Hypersecretion is common in bronchiectasis, resulting in mucus plugging within dilated airways. On CT, these appear as branching tubular opacities, sometimes called "tree-in-bud" patterns when affecting smaller airways. Retained secretions increase infection risk and contribute to symptom burden.

3. Mosaic Attenuation and Air Trapping

This pattern shows patchy areas of differing lung density, reflecting regional differences in airflow. Low-attenuation regions indicate air trapping, often due to small airway obstruction. When combined with bronchial dilation, mosaic attenuation strengthens the diagnostic confidence.

4. Bronchial Artery Enlargement

Chronic inflammation stimulates vascular remodeling, leading to hypertrophy of the bronchial arteries. Enlarged vessels are visible on CT angiography and may correlate with hemoptysis risk, especially in patients with extensive disease.

Accurate interpretation of these CT findings enables clinicians to distinguish bronchiectasis from other chronic respiratory conditions such as COPD or asthma. A comprehensive radiological evaluation not only confirms the diagnosis but also helps assess disease severity, distribution, and potential complications—guiding personalized treatment strategies including antibiotics, airway clearance techniques, and, in select cases, surgical intervention.