Can you summarize the latest CT interpretation findings for interstitial lung disease (ILD)?

Latest CT Interpretation for Interstitial Lung Disease

The 2025 American Thoracic Society guidelines establish that ILD on CT is defined by fibrotic abnormalities (honeycombing and/or reticulation with traction bronchiectasis) involving ≥5% of total lung volume, progressive fibrotic changes on serial imaging, or presence of major fibrotic patterns including UIP/probable UIP, fibrotic hypersensitivity pneumonitis, or fibrotic NSIP 1.

Key CT Imaging Criteria for ILD Diagnosis

The latest guidelines distinguish between interstitial lung abnormalities (ILAs) and true ILD based on extent of involvement:

• ILAs: Abnormalities affecting >5% of any lung zone
• ILD: Fibrotic changes involving ≥5% of total lung volume (not just a single zone)

This distinction captures more diffuse disease involvement and enhances diagnostic specificity 1.

Major Fibrotic Patterns to Identify

When interpreting CT for ILD, specifically look for these three major fibrotic patterns 1:

1. UIP/Probable UIP pattern

   • Subpleural and basal predominant distribution
   • Honeycombing with or without peripheral traction bronchiectasis
   • Reticular opacities

2. Fibrotic Hypersensitivity Pneumonitis

   • Upper and mid-lung predominance possible
   • Mosaic attenuation with air trapping
   • Centrilobular nodules may be present

3. Fibrotic NSIP

   • Bilateral, symmetric ground-glass opacities
   • Reticular changes with traction bronchiectasis
   • Subpleural sparing (unlike UIP)

Critical Imaging Features for Assessment

Primary Fibrotic Features

• Honeycombing: Clustered cystic airspaces with thick walls
• Reticulation with traction bronchiectasis: Linear opacities causing bronchial distortion (see Figure 2 description in guidelines showing bilateral subpleural reticular opacities with traction bronchiectasis/bronchiolectasis) 1
• Traction bronchiolectasis: Dilated small airways from surrounding fibrosis

Distribution Patterns Matter

Visual assessment should note:

• Craniocaudal distribution: Upper vs. lower lung predominance
• Axial distribution: Subpleural, peribronchovascular, or diffuse
• Symmetry: Bilateral symmetric vs. asymmetric involvement

Progressive Pulmonary Fibrosis (PPF) Detection

For monitoring progression, serial CT comparison is now a stand-alone criterion for reduced transplant-free survival 2. Look for:

• Increasing extent of reticulation
• New or worsening traction bronchiectasis
• Development or progression of honeycombing
• Increasing ground-glass opacities in fibrotic areas

The 2022 ATS/ERS/JRS/ALAT guidelines define PPF when radiologic progression occurs within the past year, making serial imaging comparison essential 2.

Quantitative CT Considerations

While visual assessment remains the standard, quantitative CT (QCT) is emerging as an important adjunct 3, 4:

• High-attenuation areas (HAA): Increased on both inspiratory and expiratory images in ILD
• Functional small airway disease (fSAD): Decreased in fibrotic ILD
• Pulmonary vascular parameters: Vessel counts and cross-sectional areas decrease with ILD progression 5

Important caveat: QCT estimates may differ from visual assessment, and standardized thresholds are still being established 1. Visual scoring remains the primary method for clinical decision-making.

Practical Interpretation Algorithm

When reviewing chest CT for suspected ILD:

1. Estimate total lung volume involvement: Is fibrosis ≥5% of total lung volume?
2. Identify specific patterns: Which major fibrotic pattern (UIP, fibrotic HP, fibrotic NSIP)?
3. Assess distribution: Subpleural vs. peribronchovascular; upper vs. lower lung
4. Document key features: Honeycombing, reticulation, traction bronchiectasis/bronchiolectasis
5. Compare to prior imaging: Any progression in extent or severity?

Patterns NOT Included in ILD Definition

The guidelines specifically exclude certain patterns when found in isolation without symptoms or physiologic abnormalities 1:

• Pleuroparenchymal fibroelastosis
• Lymphoid interstitial pneumonia
• Smoking-related interstitial fibrosis

These have lower likelihood of progression and should not be classified as ILD unless accompanied by clinical or physiologic criteria.

Context-Specific Patterns

CTD-ILD Considerations

For connective tissue disease-related ILD, specific patterns help distinguish etiology 6:

• IIM-ILD: NSIP in upper lung fields + organizing pneumonia (OP) in middle/lower fields (OR 20.51 for this combination)
• SSc and RA-ILD: More variable patterns requiring integration with clinical context 7

Biomass Smoke Exposure

A distinct pattern has been characterized 8:

• Reticulation (70%), ground-glass opacities (65%), mosaic attenuation (65%)
• Patchy and peribronchial distribution (not subpleural like IPF)
• Less honeycombing than IPF
• Lacks centrilobular nodularity of hypersensitivity pneumonitis

Common Pitfalls to Avoid

1. Confusing ILA with ILD: ILA affects >5% of a lung zone; ILD requires ≥5% of total lung volume
2. Missing traction bronchiectasis: This is the key feature distinguishing fibrotic from non-fibrotic disease
3. Overlooking progression: Serial comparison is now a diagnostic criterion itself—always compare to prior studies
4. Misclassifying distribution: Subpleural vs. peribronchovascular distribution helps distinguish UIP from other patterns
5. Ignoring clinical context: Even classic UIP pattern can occur with <5% involvement; integrate with symptoms and physiology 1