What are the main differences in computed tomography (CT) scans between a nodule and a cavity, and what are the identifiable characteristics for bronchiectasis?

CT Differentiation Between Pulmonary Nodules and Cavities, and Characteristics of Bronchiectasis

The main difference between a pulmonary nodule and a cavity on CT is that a nodule appears as a solid or partially solid opacity, while a cavity is characterized by an air-containing lesion with a well-defined wall. 1

Pulmonary Nodule Characteristics on CT

• Definition: A rounded or irregular opacity, well or poorly defined, measuring up to 3 cm in diameter
• Appearance:
  • Solid, part-solid, or ground-glass opacity
  • May have smooth, lobulated, or irregular/spiculated margins
  • No central air content (distinguishing it from a cavity)
• Key features to assess:
  • Size (most important predictor of malignancy)
  • Margins (spiculated or irregular margins suggest malignancy)
  • Density (solid, part-solid, or ground-glass)
  • Calcification patterns (diffuse, central, laminated, and "popcorn" patterns suggest benignity)
  • Growth rate (doubling time between 20-400 days suggests malignancy) 1

Cavity Characteristics on CT

• Definition: An air-containing lesion with a well-defined wall
• Appearance:
  • Air-filled space within pulmonary consolidation, mass, or nodule
  • Wall thickness varies (thick walls suggest malignancy or infection)
  • May contain air-fluid levels
• Key features to assess:
  • Wall thickness (thick and irregular walls suggest malignancy)
  • Wall regularity (irregular inner contours suggest malignancy)
  • Surrounding parenchymal changes
  • Associated findings (e.g., satellite lesions, lymphadenopathy) 2

Bronchiectasis Characteristics on CT

Bronchiectasis is permanent irreversible dilatation of the airways that can be readily identified on CT with several characteristic findings:

• Primary diagnostic criteria:

  • Bronchial diameter exceeding that of the adjacent pulmonary artery (bronchoarterial ratio >1) 3, 4
  • Lack of normal tapering of bronchi as they extend toward the lung periphery 3, 4
  • Visualization of bronchi within 1 cm of the pleural surface 3, 4
  • Bronchi abutting the mediastinal pleura 4

• Additional findings:

  • Bronchial wall thickening due to peribronchial fibrosis 5
  • Mucus-filled dilated bronchi (sometimes with air-fluid levels) 5, 3
  • Mosaic perfusion pattern (reflecting air trapping) 3
  • Centrilobular nodules or branching areas of soft-tissue attenuation (tree-in-bud pattern) 3
  • Bronchiolectasis (dilatation of bronchioles) 3

• Morphological patterns:

  • Cylindrical: Uniform dilatation with smooth tapering
  • Varicose: Irregular dilatation with alternating areas of constriction
  • Cystic: Balloon-like dilatation with potential saccular formations 2

Clinical Significance and Pitfalls

• Nodule vs. Cavity: Distinguishing between these is critical as cavitary lesions may represent more advanced disease processes, including malignancy with central necrosis, tuberculosis, fungal infections, or vasculitis 2

• Bronchiectasis: Represents irreversible airway damage, often associated with chronic infection, inflammation, or congenital conditions. Severity on CT correlates with measures of airflow obstruction and prognosis 2

• Common pitfalls:

  • Pseudonodules (approximately 20% of suspicious nodules on chest radiographs) can be mistaken for true nodules 1
  • Emphysematous blebs may be confused with cavities or bronchiectasis but typically have no definable wall thickness 5
  • Normal bronchi in the peripheral third of the lung can sometimes be mistaken for mild bronchiectasis 4

High-resolution CT (1-1.5mm slices) without contrast is the optimal imaging modality for detailed evaluation of these pulmonary findings, providing superior characterization compared to conventional radiography 1, 2.