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Radiology of Interstitial Lung Disease (ILD): A Complete Guide

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Part 1: Framework - How to Approach ILD Radiology

Imaging Modality Hierarchy

• Increases diagnostic accuracy from 27% (clinical alone) to 61% when added to clinical data and CXR
• Distinguishes reversible (ground-glass, active inflammation) from irreversible (honeycombing, fibrosis) changes
• Guides bronchoalveolar lavage (BAL) and biopsy site selection
• Monitors treatment response
• In UIP/IPF, a typical pattern on HRCT can obviate surgical lung biopsy

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Part 2: The HRCT Pattern Dictionary

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Part 3: Idiopathic Interstitial Pneumonias (IIPs)

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1. Idiopathic Pulmonary Fibrosis (IPF) / Usual Interstitial Pneumonia (UIP)

The 4 HRCT Categories (ATS/ERS/JRS/ALAT 2018 + Fleischner Society 2018):

• Predominantly subpleural, bibasal reticular pattern
• Honeycombing (thick-walled cysts in stacked layers)
• Traction bronchiectasis and bronchiolectasis
• The disease "creeps" up the periphery in a "propeller blade" distribution on sagittal images
• GGO is NOT dominant; when present, it is surrounded by traction bronchiectasis (indicating fibrosis, not activity)
• No features suggesting alternative diagnosis

• Subpleural bibasal reticular pattern
• Peripheral traction bronchiectasis/bronchiolectasis
• No honeycombing
• Reflects definite/probable UIP pathology in 82-94% of cases

• Subtle reticulation; may have mild GGO
• No distribution that fits typical/probable
• No features inconsistent with UIP

• Upper or mid-lung predominant fibrosis
• Peribronchovascular predominance
• Extensive GGO (>extent of reticulation)
• Profuse micronodules, discrete cysts away from honeycombing zones
• Diffuse air trapping (mosaic pattern)
• Consolidation in bronchopulmonary segment/lobe

• True honeycombing: thick-walled cysts, subpleural, stacked in layers, always with other fibrotic features (traction bronchiectasis, architectural distortion)
• Paraseptal emphysema: thin-walled, single layer subpleural, no fibrotic features
• LAM cysts: randomly scattered, normal lung between cysts, no subpleural predominance, no fibrosis
• Cystic bronchiectasis: centrally located, along vessels, tubular shape

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2. Nonspecific Interstitial Pneumonia (NSIP)

• Bilateral symmetric ground-glass attenuation - the dominant finding
• Peripheral, subpleural, lower-lobe predominance
• Subpleural sparing - a characteristic and distinctive feature (unlike UIP)
• Irregular lines and mild reticulation
• Occasional consolidation
• Traction bronchiectasis when fibrotic NSIP
• Honeycombing rare (if present, indicates fibrotic NSIP, worse prognosis)

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3. Cryptogenic Organizing Pneumonia (COP)

• Patchy bilateral consolidation - most characteristic
• Distribution: subpleural or peribronchial (peribronchovascular)
• Air bronchograms within consolidation
• Lower lobe predominance
• "Reversed halo sign" (atoll sign): crescent or ring of consolidation around central GGO - highly specific for COP
• Perilobular pattern: band-like consolidation along lobular borders
• Migration of lesions over time ("migratory infiltrates")
• GGO with traction bronchiectasis may be present in fibrotic COP

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4. Acute Interstitial Pneumonia (AIP) / Diffuse Alveolar Damage (DAD)

• Exudative phase (days 1-7): Bilateral diffuse GGO and consolidation, often with lobular sparing (geographic distribution)
• Organizing phase (weeks 2-4): Traction bronchiectasis develops within GGO (hallmark of transition to fibrosis)
• Dependent consolidation common
• Anterior-posterior density gradient (dependent vs. non-dependent)
• Architectural distortion in fibrotic phase

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5. Respiratory Bronchiolitis-ILD (RB-ILD)

• Diffuse, patchy ground-glass opacification due to macrophage accumulation
• Poorly defined centrilobular nodules (fluffy, low-attenuation)
• Upper lobe predominance (bilateral, symmetric)
• Centrilobular emphysema (related to smoking)
• Areas of air trapping
• Interlobular septal thickening and interstitial fibrosis may occur but are not dominant
• Normal CXR in up to 20% of RB-ILD patients

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6. Desquamative Interstitial Pneumonia (DIP)

• Extensive ground-glass opacification - dominant feature
• Distribution: lower zone, peripheral, may be patchy or geographic
• Associated reticulation (mild)
• Small cysts may be present within GGO
• Features of established fibrosis (architectural distortion, mild bronchiectasis) in some patients
• Normal CXR in up to 25% of cases

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7. Lymphoid Interstitial Pneumonia (LIP)

• Diffuse GGO (dominant)
• Thin-walled cysts (scattered, often bilateral) - highly characteristic
• Centrilobular nodules
• Thickened interlobular septa and bronchovascular wall thickening
• Lower lobe predominance

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Part 4: Non-Idiopathic ILD

8. Sarcoidosis

• Stage 0: Normal
• Stage I: Bilateral hilar lymphadenopathy (BHL) only
• Stage II: BHL + parenchymal infiltrates
• Stage III: Parenchymal infiltrates only (no BHL)
• Stage IV: Pulmonary fibrosis

• Perilymphatic nodules (1-5mm) - along bronchovascular bundles, pleura (including fissures), and interlobular septa - this distribution is the key
• "Galaxy sign": smaller nodules cluster around a large central nodule - highly specific for sarcoidosis
• Irregular and beaded interfaces
• Larger ill-defined nodules ± air bronchograms
• Patchy GGO
• Interlobular septal thickening
• Air trapping - correlates with small airways disease on PFTs
• Advanced disease: upper/mid-zone fibrosis, traction bronchiectasis, bullae, ring shadowing
• Progressive massive fibrosis (PMF) - resembling silicosis - in end-stage

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9. Hypersensitivity Pneumonitis (HP)

• Bilateral GGO and consolidation
• Often resolves spontaneously on antigen removal

• Diffuse poorly defined centrilobular nodules (3-5mm)
• Patchy bilateral GGO
• Air trapping on expiratory CT (lobular or panlobular) - very characteristic
• Mosaic perfusion - areas of normal and reduced attenuation
• Mid and upper lobe predominance
• Head-cheese sign: combination of GGO, normal lung, and air trapping creating three different densities

• Reticulation and fibrosis
• Upper/mid lobe predominance (differs from IPF which is lower lobe)
• Traction bronchiectasis
• May develop honeycombing - can mimic UIP
• Peribronchovascular distribution (not purely subpleural like UIP)
• GGO and centrilobular nodules often persist alongside fibrosis (important clue)

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10. Connective Tissue Disease-ILD (CTD-ILD)

• Most common pattern: UIP (more than NSIP, unlike other CTDs)
• HRCT indistinguishable from idiopathic UIP/IPF
• Also: bronchiectasis (up to 30%), obliterative bronchiolitis, follicular bronchiolitis, COP pattern, pleural effusions
• Necrobiotic (rheumatoid) nodules: well-defined, peripheral, may cavitate

• Most common ILD pattern: fibrotic NSIP
• Subpleural basal GGO with reticulation, subpleural sparing
• Dilated esophagus on HRCT - a helpful diagnostic clue
• Pulmonary hypertension disproportionate to fibrosis extent

• Mixed pattern: GGO, reticular, consolidation
• Both NSIP and COP patterns occur
• "Mechanic's hands," elevated CK, anti-Jo-1 in some

• LIP pattern (GGO + thin-walled cysts)
• Also: follicular bronchiolitis, NSIP

• Acute lupus pneumonitis (bilateral GGO/consolidation)
• Pleuritis/effusions common
• Shrinking lung syndrome: small lung volumes, elevated diaphragm

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11. Langerhans Cell Histiocytosis (LCH / PLCH)

• Upper and mid-lung predominance (spares costophrenic angles)
• Bilateral irregular/bizarre-shaped cysts - key feature; cysts vary in size and shape (not all round)
• Centrilobular nodules (early disease) that cavitate and form cysts (later)
• Normal lung volumes or hyperinflation
• Pneumothorax is a recognized complication

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12. Lymphangioleiomyomatosis (LAM)

• Bilateral, diffuse, thin-walled, round cysts - uniformly distributed throughout the lung
• Normal lung intervening between cysts (unlike UIP honeycombing)
• No subpleural predominance
• No fibrosis
• Cysts increase in number/size over time
• Pleural effusion (chylothorax) may occur
• Pneumothorax common

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13. Asbestosis

• Bilateral lower-lobe fibrosis - subpleural, basal (similar to UIP)
• Reticular pattern, traction bronchiectasis, honeycombing in advanced disease
• Subpleural dots and lines - early characteristic finding
• Pleural plaques - calcified or non-calcified, diaphragmatic/posterolateral
• Pleural thickening (diffuse)
• Rounded atelectasis ("crow's feet" - curvilinear bands converging on pleura)

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14. Silicosis / Coal Worker's Pneumoconiosis (CWP)

• Upper lobe nodules (1-10mm) - bilateral, well-defined
• Perilymphatic distribution (like sarcoidosis)
• Eggshell calcification of hilar/mediastinal nodes (pathognomonic)

• Conglomerate masses (>1cm, usually >2cm) in upper lobes
• "Egg-in-a-cup" or PMF masses migrating toward hilum
• Associated emphysema

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15. Eosinophilic Pneumonia (Chronic)

• Peripheral consolidation and GGO - "photographic negative of pulmonary edema"
• Upper/mid-lung zone predominance with peripheral distribution
• Recurrent or migratory lesions (unlike most ILDs)
• Air bronchograms within consolidation
• Pleural effusion in ~10%

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16. Drug-Induced ILD

• Amiodarone: high-density consolidation (due to iodine in drug), peripheral GGO, NSIP pattern
• Methotrexate: hypersensitivity pattern, GGO, consolidation
• Chemotherapy (bleomycin, cyclophosphamide): COP pattern, NSIP pattern
• Nitrofurantoin: pleural effusion + parenchymal infiltrates

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Part 5: Chest X-Ray Pattern → Differential Diagnosis

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Part 6: Diagnostic Approach Summary

Step 1: Clinical context
   - Age, sex, smoking history, occupation, drug history, CTD symptoms
   - Pace of onset (acute, subacute, chronic)

Step 2: CXR
   - Lung volumes (decreased vs preserved)
   - Distribution (upper/lower, central/peripheral)
   - Pattern (nodular, reticular, consolidation, mixed)
   - Pleural changes, adenopathy

Step 3: HRCT (1mm, prone and expiratory views)
   - Classify dominant pattern (GGO, reticular, honeycomb, nodular, consolidation, cystic)
   - Assess distribution (upper/lower, central/peripheral/subpleural, axial)
   - Look for specific signs (perilymphatic nodules, galaxy sign, reversed halo, subpleural sparing)

Step 4: Multidisciplinary Discussion
   - Radiologist + Pulmonologist + Pathologist
   - Correlate with serology, BAL, PFTs
   - Decide if biopsy is needed or if HRCT pattern is diagnostic

Step 5: Biopsy (if needed)
   - Site guided by HRCT (most active disease, avoid honeycombing/end-stage areas)

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Quick Reference: ILD Pattern on HRCT

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ILD Radiology: Complete Summary with Tables and Flowcharts

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FLOWCHART 1: Overall Diagnostic Approach to ILD

┌─────────────────────────────────────────────────────────────────┐
│        INTERSTITIAL LUNG DISEASE SUSPECTED                       │
│    (dyspnoea, cough, bibasal crackles, restrictive PFTs)        │
└──────────────────────────┬──────────────────────────────────────┘
                           │
                    CHEST X-RAY
                           │
        ┌──────────────────┼──────────────────┐
        ▼                  ▼                  ▼
   Normal (10%)     Reticular/nodular    Consolidation/GGO
        │           lower zones          bilateral
        │                  │                  │
        └──────────────────┼──────────────────┘
                           │
                    ┌──────▼──────┐
                    │  HRCT CHEST │  (1mm sections, prone,
                    │  (KEY STEP) │   expiratory views)
                    └──────┬──────┘
                           │
         ┌─────────────────┼──────────────────┐
         ▼                 ▼                  ▼
   INTERSTITIAL      ALVEOLAR FILLING    COMBINED
   PATTERNS          PATTERNS            PATTERNS
         │                 │                  │
   ┌─────┴─────┐      ┌────┴────┐             │
   Fibrotic    Non-   OP  RB-ILD  AMP         MDD
              Fibrotic rare AFPs
         │
   ┌─────┼──────┐
  UIP   NSIP   BIP   PPFE  LIP  DAD
         │
         ▼
   MULTIDISCIPLINARY DISCUSSION (MDD)
   Clinical + Radiology + Pathology + Lab
         │
         ▼
   ┌─────────────────────────────────────────────┐
   │ DIAGNOSTIC CONFIDENCE                        │
   │  ≥90% → Confident Diagnosis                  │
   │  51-89% → Provisional Diagnosis              │
   │  <50% → Unclassifiable ILD                   │
   └─────────────────────────────────────────────┘

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FLOWCHART 2: HRCT Pattern Classification (2025 ERS/ATS Schema)

              HRCT Abnormality Identified
                          │
          ┌───────────────┴───────────────┐
          ▼                               ▼
   INTERSTITIAL                    ALVEOLAR FILLING
   DISORDERS                       DISORDERS
          │                               │
   ┌──────┴──────┐               ┌────────┴────────┐
   │             │               │                 │
Fibrotic    Non-fibrotic      Common              Rare
   │             │               │                 │
┌──┴──┐    ┌────┴────┐     ┌────┴────────┐  ┌────┴────────┐
UIP   NSIP  NSIP   BIP    OP  RB-ILD  AMP  AEP CEP PAP
NSIP  BIP  (cell) (non-        │             Lipoid pn.
BIP   LIP  fibr)  fibrot)
PPFE  DAD                   Subcategories of OP:
                            Multifocal/focal
                            AFOP, CiOP

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TABLE 1: Key HRCT Patterns and Their Meaning

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TABLE 2: IIP/ILD Classification - 2025 ERS/ATS Update

Key 2025 changes: "DIP" renamed → Alveolar Macrophage Pneumonia (AMP); "HP pattern" → Bronchiolocentric Interstitial Pneumonia (BIP); new subcategories of OP added (AFOP, CiOP); PPFE promoted from "rare" to "major" pattern

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TABLE 3: Radiological Features of All Major ILD Types (Master Reference Table)

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FLOWCHART 3: UIP/IPF Decision Tree (2018 ATS/ERS/JRS/ALAT + Fleischner 2018)

Patient with suspected IPF
(age >60, male, smoker, no identified cause)
                │
                ▼
           HRCT CHEST
                │
    ┌───────────┼───────────────────┐
    ▼           ▼                  ▼
TYPICAL UIP  PROBABLE UIP    INDETERMINATE    ALTERNATIVE
                               for UIP         DIAGNOSIS
    │           │                  │               │
Subpleural  Subpleural       Subtle reticular  Nodules, cysts,
bibasal     bibasal          ± mild GGO       consolidation,
reticular   reticular        No dominant       upper predominant
+           + traction       pattern           peribronchovascular
HONEYCOMB  bronchiectasis
+ traction  NO honeycomb
    │           │                  │               │
    ▼           ▼                  ▼               ▼
No biopsy    Integrate        Consider        Consider
needed      clinical prob.    biopsy         alternative
(>90%       (82-94% UIP      ↓               diagnosis
confidence) at histology)
                │           SLB / TBLC      (HP, NSIP, CTD,
            If low prob.    shows UIP?       sarcoid, etc.)
            → consider      ↓
            biopsy          Yes → IPF
                            No → Alternative

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CT IMAGE GALLERY

UIP/IPF - Typical Pattern

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UIP/IPF - Probable Pattern

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Honeycombing vs Mimics - Critical Comparison

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DIP (now AMP - Alveolar Macrophage Pneumonia)

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Sarcoidosis

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TABLE 4: Chest X-Ray Pattern → Differential Diagnosis

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TABLE 5: Radiological Features of IIPs (2025 ERS/ATS Table 2 - Ryerson et al.)

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TABLE 6: Combined Patterns and Clinical Differential Diagnoses

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TABLE 7: Cystic Lung Diseases - Differential Diagnosis

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TABLE 8: Disease Behaviour and Prognosis

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FLOWCHART 4: Honeycombing vs Mimic Decision Tree

SUBPLEURAL CYSTS SEEN ON HRCT
              │
    ┌─────────┴──────────┐
    ▼                    ▼
Thick-walled?          Thin-walled?
Stacked layers?              │
Other fibrosis signs?   ┌────┴────────────────┐
    │                   ▼                     ▼
   YES              Diffuse,           Subpleural,
    │               uniform,           single layer,
    ▼               round cysts        no fibrosis
TRUE                    │                    │
HONEYCOMBING            ▼                    ▼
(UIP/IPF,           ┌───┴───┐           Paraseptal
CTD-ILD,            │       │           Emphysema
asbestosis)        LAM    LIP/LCH
                  (women) (smokers/
                           CTD)

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Quick Reference Card: HRCT Spot Diagnosis

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