What does interstitial lung disease (ILD) look like on imaging?

Imaging Appearance of Interstitial Lung Disease

Interstitial lung disease appears on high-resolution CT (HRCT) as bilateral abnormalities involving >5% of total lung volume, characterized by ground-glass opacities, reticular patterns, traction bronchiectasis, and/or honeycombing, with distribution patterns that help distinguish specific ILD subtypes. 1

Key Imaging Features by Pattern

Usual Interstitial Pneumonia (UIP) Pattern

The UIP pattern is the hallmark of idiopathic pulmonary fibrosis and appears with specific features:

• Subpleural and basal predominant distribution with patchy involvement 1
• Honeycombing consisting of clustered cystic airspaces with thick walls, typically in subpleural regions 1
• Reticular opacities with traction bronchiectasis indicating architectural distortion 1
• Absence of features inconsistent with UIP, such as predominant ground-glass opacities, mosaic attenuation, or centrilobular nodules 1

Fibrotic Hypersensitivity Pneumonitis Pattern

Fibrotic HP demonstrates a distinct constellation of findings:

• Profuse poorly defined ground-glass centrilobular nodules scattered throughout the lungs 1
• Three-density sign showing areas of normal lung, ground-glass opacity, and air trapping on expiratory imaging 1
• Mosaic attenuation pattern with air trapping on expiratory sequences 1
• Upper and mid-lung zone predominance in many cases, though distribution can be variable 1

Interstitial Lung Abnormalities (ILA) Subtypes

ILAs represent early or limited disease involving ≤5% of a lung zone and are categorized into three subtypes: 1

Nonsubpleural ILA

• Patchy ground-glass and peribronchovascular opacities predominantly in upper lung zones 1
• Minimal reticular opacities without architectural distortion 1

Subpleural Nonfibrotic ILA

• Bilateral subpleural reticulations without traction bronchiectasis or honeycombing 1
• Absence of architectural distortion distinguishing it from fibrotic disease 1

Fibrotic ILA

• Bilateral subpleural reticulations with traction bronchiolectasis indicating early fibrosis 1
• Architectural distortion present but involving <5% of total lung volume 1

Progression to Definite ILD

When imaging abnormalities meet specific thresholds, ILA transitions to definite ILD:

• Fibrotic abnormalities involving ≥5% of total lung volume by visual estimate, defined by honeycombing and/or reticulation with traction bronchiectasis 1
• Progressive fibrotic abnormality on serial chest CT demonstrating interval worsening 1
• Presence of a major fibrotic ILD pattern including UIP/probable UIP, fibrotic HP, or fibrotic NSIP regardless of extent 1

Imaging Modality Recommendations

HRCT as Gold Standard

HRCT is the definitive imaging modality for ILD evaluation:

• HRCT has 95.7% sensitivity and 63.8% specificity for detecting ILD with ≥20% lung involvement 2
• Chest radiography alone is insufficient, as up to 10% of ILD patients have normal chest X-rays 2
• HRCT should include inspiratory prone images and supine end-expiratory imaging to distinguish true abnormalities from dependent atelectasis 1, 2

Role of Other Modalities

• CT with IV contrast serves no purpose in suspected ILD evaluation 1
• MRI has limited utility and has not been validated for routine ILD diagnosis 1
• FDG-PET/CT is not indicated for initial ILD evaluation 1

Common Pitfalls and Caveats

Several imaging findings require careful interpretation to avoid misdiagnosis:

• Dependent ground-glass opacities on supine imaging may represent atelectasis rather than true pathology—confirm persistence on prone imaging 1, 3
• Isolated reticulation without traction bronchiectasis is a risk factor for progression but insufficient for ILD diagnosis unless progression is documented 1
• Bronchiolectasis alone (dilation of bronchioles within central portions of secondary pulmonary lobule) is less prognostically informative and not included in ILD definition 1
• UIP pattern can occur in non-IPF diagnoses including connective tissue disease-ILD, fibrotic HP, and asbestosis—clinical context is essential 1

Distinguishing Features for Specific Diagnoses

Additional imaging clues help identify underlying etiology:

• Exuberant honeycombing, anterior upper lobe involvement, or straight-edge sign suggest connective tissue disease-ILD 1
• Bilateral pleural plaques indicate asbestos exposure 1
• Dilated esophagus or osseous erosions suggest systemic sclerosis 1
• Well-demarcated preserved lobules with intervening ground-glass opacities favor hypersensitivity pneumonitis over IPF 1