Bronchiectasis on CT Imaging: Understanding Types, Causes, and Diagnostic Criteria

When evaluating lung conditions using computed tomography (CT), bronchiectasis stands out as a key finding characterized by abnormal and permanent dilation of the bronchial airways. This condition manifests in three primary forms on imaging: cylindrical, cystic, and varicose (mixed-type). Each type presents distinct visual patterns on CT scans, helping radiologists and pulmonologists make accurate diagnoses. Cylindrical bronchiectasis appears as uniformly widened bronchi, while cystic types show balloon-like dilations filled with mucus or air. The mixed or varicose form combines irregular thickening and thinning of the bronchial walls, resembling a beaded string.

Understanding the Mechanisms Behind Bronchial Dilation

The development of bronchiectasis is often linked to two main pathophysiological processes: increased internal pressure within the airways and external traction forces. Internal pressure buildup can result from chronic airway obstruction, often due to mucus plugging or recurrent infections, which gradually weakens the bronchial walls. On the other hand, external traction occurs when surrounding lung tissues undergo fibrosis—commonly seen in conditions such as pulmonary tuberculosis, post-inflammatory scarring, or interstitial lung disease. As scar tissue contracts, it pulls on adjacent bronchi, distorting their structure and leading to localized enlargement.

Anatomical Perspective: The Tree-Like Structure of the Airways

The respiratory system resembles an upside-down tree, with the trachea acting as the trunk, branching into larger bronchi (major limbs), then progressively smaller bronchioles (twigs). Normally, these airways taper smoothly from proximal (closer to the lungs' center) to distal (toward the outer edges). In healthy individuals, each successive branch becomes narrower. However, in bronchiectasis, this pattern reverses—distal segments become abnormally wider than their proximal counterparts, a hallmark sign visible on high-resolution CT (HRCT).

Key Radiological Criteria for Diagnosing Bronchiectasis

Radiologists rely on specific imaging criteria to confirm bronchiectasis. One widely accepted rule is the "signet ring sign," where a dilated bronchus appears larger in diameter than the accompanying pulmonary artery, resembling a ring with a stone. This indicates pathological widening since, under normal circumstances, the artery and bronchus are roughly equal in size at corresponding levels. Additionally, lack of normal tapering along the bronchial course and visualization of bronchi extending all the way to the pleural surface further support the diagnosis.

Visualizing Bronchial Abnormalities on CT Scans

In both cystic and cylindrical forms, CT imaging reveals airways that fail to narrow appropriately. Instead, they maintain or increase in diameter as they extend outward. These structural changes are not only diagnostic but also help assess disease severity and distribution. Modern multidetector CT scanners offer detailed cross-sectional views, enabling clinicians to identify early-stage bronchiectasis before significant lung function decline occurs.

Overall, recognizing bronchiectasis on CT involves identifying abnormal bronchial widening, loss of normal tapering, and characteristic signs like the signet ring appearance. Whether caused by chronic infection, immune dysfunction, or post-fibrotic remodeling, timely detection through advanced imaging plays a crucial role in managing respiratory health and improving patient outcomes across diverse clinical settings.