How to differentiate between perilymphatic and centrilobular micronodules on a computed tomography (CT) scan?

Differentiating Perilymphatic and Centrilobular Micronodules on CT Scan

The key to differentiating perilymphatic from centrilobular micronodules on CT scan is to identify their specific anatomical distribution patterns within the secondary pulmonary lobule, with perilymphatic nodules following lymphatic pathways along pleural surfaces, interlobular septa, and bronchovascular bundles, while centrilobular nodules are located in the center of the secondary pulmonary lobule with sparing of the pleural surfaces.

Anatomical Distribution Patterns

Perilymphatic Micronodules

• Location characteristics:
  • Distributed along lymphatic pathways
  • Found along pleural surfaces (subpleural)
  • Present along interlobular septa
  • Visible along bronchovascular bundles
  • Often have polygonal or linear arrangement
  • Typically contact the pleural surface
  • May have upper lobe predominance (especially in sarcoidosis)

Centrilobular Micronodules

• Location characteristics:
  • Centered within the secondary pulmonary lobule
  • Separated from the pleural surface by several millimeters
  • Often have a tree-in-bud appearance in infectious/inflammatory conditions
  • Typically spare the subpleural regions and interlobular septa
  • May show ground-glass appearance in hypersensitivity pneumonitis
  • Often diffuse but can have upper lobe predominance in hypersensitivity pneumonitis

Diagnostic Algorithm

1. Assess relationship to pleural surfaces:

   • Perilymphatic: Contact with pleural surfaces
   • Centrilobular: Separated from pleural surfaces by several millimeters

2. Evaluate distribution along interlobular septa:

   • Perilymphatic: Nodules align along visible interlobular septa
   • Centrilobular: No relationship to interlobular septa

3. Examine relationship to bronchovascular bundles:

   • Perilymphatic: Nodules follow bronchovascular bundles
   • Centrilobular: Centered around terminal bronchioles but not necessarily following larger airways

4. Look for associated findings:

   • Perilymphatic: Often associated with lymphadenopathy (sarcoidosis)
   • Centrilobular: May be associated with mosaic attenuation and air-trapping (hypersensitivity pneumonitis)

Clinical Significance and Associated Diseases

Perilymphatic Pattern

• Common causes:
  • Sarcoidosis
  • Lymphangitic carcinomatosis
  • Silicosis/pneumoconiosis
  • Lymphoproliferative disorders
  • Amyloidosis

Centrilobular Pattern

• Common causes:
  • Hypersensitivity pneumonitis
  • Respiratory bronchiolitis
  • Infectious bronchiolitis
  • Diffuse panbronchiolitis
  • Endobronchial spread of tuberculosis
  • Pulmonary Langerhans cell histiocytosis

Technical Considerations

• Use thin-section CT (1.5 mm) for optimal characterization 1
• Evaluate in multiple planes (axial, coronal, sagittal) to better appreciate the distribution
• Consider expiratory images to detect air-trapping, which is common in centrilobular diseases like hypersensitivity pneumonitis 1
• Standardize acquisition and reconstruction protocols to ensure accurate assessment 1

Pitfalls to Avoid

1. Inadequate inspiration can increase lung attenuation and mimic ground-glass appearance 1
2. Thick CT sections may obscure the fine details needed to distinguish between patterns
3. Overlapping patterns can occur in some diseases
4. Advanced fibrosis can distort the normal lung architecture, making pattern recognition difficult
5. Profuse micronodules may be difficult to characterize when they become confluent

Practical Tips

• The "three-density sign" (normal lung, ground-glass areas, and decreased attenuation areas) on inspiratory CT is highly suggestive of hypersensitivity pneumonitis with centrilobular nodules 1
• Perilymphatic nodules often have a more sharply defined appearance compared to the hazier appearance of centrilobular nodules
• When both patterns coexist, focus on the predominant pattern and associated findings
• Consider the clinical context, as certain diseases have characteristic distributions (e.g., sarcoidosis typically shows perilymphatic nodules with upper lobe predominance)

By systematically evaluating the distribution of micronodules in relation to the secondary pulmonary lobule structures, radiologists can accurately differentiate between perilymphatic and centrilobular patterns, which is crucial for narrowing the differential diagnosis and guiding appropriate management 2.