Lungs

Name the Structures
Which of these structures are considered to be “small airways”?

2. Findings and Diagnosis

Definition of Small Airways

• Small airways are non-cartilaginous airways with a diameter of ≤2 mm.
• Begin at the terminal bronchioles, the last purely conducting airways.
• Include the respiratory bronchioles, which transition to the gas exchange zone.
• Are located within the secondary lobule, where each terminal bronchiole serves as the central feeding airway.
• Play a key role in ventilation and resistance to airflow.
• Typically invisible on CT unless diseased, appearing as bronchiolectasis, tree-in-bud, or air trapping in small airway diseases.

Small Airways: Components and CT Characteristics

Component	Anatomic Characteristics	CT Appearance When Abnormal
Terminal Bronchiole	Last purely conducting airway, centrally located in the secondary lobule	Can be seen if bronchiolectatic or thickened due to inflammation
Respiratory Bronchiole	First airway with alveoli in its walls, transitional zone to gas exchange	Not directly visible but can appear in tree-in-bud pattern mosaic attenuation and air trapping, ground glass attenuation exemplified in bronchiolitis
Alveolar Ducts	Lead to alveolar sacs, part of the acinus	Not seen directly, but can show air-trapping or interstitial changes
Alveolar Sacs/Alveoli	Primary sites of gas exchange	Visible in emphysema as low-density areas (air spaces)

 

CT Manifestations of Small Airway Disease

CT Finding	Pathophysiology	Association with Small Airway Disease
Air Trapping	Incomplete emptying of lung units due to bronchiolar obstruction or narrowing	Seen in constrictive bronchiolitis (obliterative bronchiolitis), early COPD, hypersensitivity pneumonitis
Ground-Glass Opacity (GGO)	Increased lung attenuation due to partial alveolar filling or interstitial thickening	Seen in cellular bronchiolitis, hypersensitivity pneumonitis, and diffuse small airway inflammation
Tree-in-Bud Pattern	Mucus, pus, or debris in terminal and respiratory bronchioles	Seen in infectious bronchiolitis, aspiration, and endobronchial spread of infection
Centrilobular Nodules	Inflammatory changes in bronchioles and peribronchiolar tissue	Seen in small airways diseases like respiratory bronchiolitis, hypersensitivity pneumonitis, and infectious bronchiolitis

---

How They Relate to Small Airway Disease

1. Air Trapping

   • Best seen on expiratory CT, where affected areas fail to increase in attenuation (stay dark).
   • Occurs when inflammation, fibrosis, or mucus causes small airway narrowing or obstruction.
   • Classic for constrictive bronchiolitis (obliterative bronchiolitis), asthma, early COPD, and hypersensitivity pneumonitis.

2. Ground-Glass Opacity (GGO)

   • Unlike air trapping, GGO appears as hazy increased lung attenuation on inspiratory CT.
   • May indicate cellular inflammation within small airways and adjacent alveoli.
   • Common in cellular bronchiolitis (e.g., hypersensitivity pneumonitis, viral bronchiolitis, organizing pneumonia).

---

Key Takeaways

• Air trapping = hallmark of obstructive small airway disease (seen best on expiratory CT).
• GGO = inflammation or early damage, affecting both small airways and alveoli.
• Both findings can coexist in hypersensitivity pneumonitis and inflammatory small airway diseases.