How can lung bullae be distinguished from cysts on computed tomography (CT)?

Differentiating Lung Bullae from Cysts on CT

Bullae are sharply demarcated air-filled spaces measuring ≥1 cm with thin walls and no visible internal structures, while cysts have more defined walls (typically <2-4 mm thick) and often decrease in size on expiratory CT imaging, indicating airway communication.

Key Distinguishing Features

Wall Characteristics

• Bullae: Have barely perceptible to very thin walls, lack definable wall thickness, and have no accompanying vessels within them 1, 2
• Cysts: Have visible walls ranging from barely perceptible to 2-4 mm in thickness, with most being <2 mm 3

Size and Distribution

• Bullae: Measure ≥1 cm in diameter by definition, more common in usual emphysema, typically in upper lobes 1
• Cysts: Variable size (typically 2-5 mm but can reach 30 mm), usually round and evenly distributed throughout lungs with normal intervening parenchyma 3

Response to Respiration (Critical Differentiator)

The most reliable distinguishing feature is performing paired inspiratory and expiratory HRCT scans:

• Cysts: Decrease in size on expiration in the majority of cases, indicating communication with airways 4, 5, 6
• Bullae: Generally do not change size during different phases of respiration, though some may show minimal decrease 4, 5, 6

This respiratory change occurs because cystic lesions that communicate with airways will decompress during expiration, while bullae typically remain static 5.

Imaging Protocol Recommendations

When to Use CT

CT scanning is specifically recommended to differentiate emphysematous bullae from other cystic lesions in patients with severe bullous lung disease, as this distinction can prevent unnecessary and potentially dangerous interventions 7

Optimal Technique

• Use high-resolution CT (HRCT) with thin collimation (1 mm sections) 3
• View images using lung window settings with edge-enhancing (sharp) filter 8
• Add expiratory phase imaging when cystic lung lesions are identified to assess for size change 4, 5
• Use window level of -600 to -800 Hounsfield units to contrast abnormally low attenuation areas 1

Associated Features to Assess

Bullae Context

• Look for surrounding emphysematous changes with decreased vascular markings 1
• More common in upper lobes in typical emphysema
• Associated with hyperinflation and diaphragmatic flattening 1

Cyst Context

• Assess for underlying cystic lung disease (LAM, Langerhans cell histiocytosis, etc.)
• Look for preserved or increased lung volume 3
• Evaluate for other systemic manifestations 3

Common Pitfalls to Avoid

1. Don't rely on plain radiographs alone - they are insensitive for distinguishing these entities 9
2. Don't assume all thin-walled lucencies are bullae - many cystic diseases present similarly
3. Don't skip expiratory imaging when the distinction is unclear - this simple addition provides definitive differentiation in most cases 4, 5
4. Avoid confusing bronchiectatic cysts with bullae - bronchiectatic cysts appear as linear arrays or clusters, have visible wall thickening, and show size change with respiration 2

Clinical Significance

This distinction matters because:

• Bullae in severe bullous disease may be amenable to surgical treatment
• Cystic lung diseases require different management approaches
• Misidentifying a cyst as a bulla could lead to inappropriate aspiration attempts 7
• Some cystic lesions (particularly irregular cysts) may represent precursor lesions requiring surveillance 10