ILD Pattern Recognition
| Diagnosis | Distribution | Dominant HRCT Pattern | Key Pearls |
|---|---|---|---|
| UIP / IPF | Basal, subpleural, posterior | Honeycombing ± traction bronchiectasis; reticulation | Minimal GGO; subpleural honeycombing is hallmark. GGO > fibrosis favors alternative dx. Associations: IPF, RA, asbestosis. |
| NSIP | Basal, subpleural — with subpleural sparing | Bilateral symmetric GGO + reticulation; traction bronchiectasis | Subpleural sparing distinguishes from UIP. Rare honeycombing. Temporal homogeneity. Associations: CTD (SSc, PM/DM, Sjögren), drug toxicity. |
| HP | Upper/mid > lower; bilateral | Centrilobular GGO nodules; mosaic attenuation; air trapping | Subacute: centrilobular nodules + GGO + mosaic. Chronic/fibrotic: upper-lobe fibrosis, UIP-like or NSIP-like. Cause: organic dust, birds, mold. |
| Sarcoidosis | Upper/mid; peribronchovascular, subpleural, fissural | Perilymphatic nodules 2–5 mm; bilateral hilar + mediastinal LAD | Galaxy sign = cluster of nodules. Beaded fissures. LAD in ~90%. |
| COP | Peribronchovascular; lower lobe; subpleural | Consolidation ± GGO; migrating opacities | Reverse halo/atoll sign (~20%, highly specific). Air bronchograms. No LAD. Causes: idiopathic, post-infectious, drug, CTD. |
| DIP | Diffuse bilateral, lower lobe | Diffuse bilateral GGO | Heavy smoker (90%). Cysts may be present within GGO. |
| RB-ILD | Upper lobe, centrilobular | Centrilobular nodules (GGO); patchy GGO | Smoker. Centrilobular micronodules 2–3 mm. Improves with smoking cessation. |
| AIP / DAD | Diffuse bilateral; dependent consolidation | Bilateral consolidation + GGO; rapid progression over days | Rapid clinical decline over days–weeks. Organizing phase: reticulation, traction bronchiectasis. CT mirrors ARDS. |
| LIP | Lower lobe; bilateral | GGO + thin-walled perivascular cysts | Cysts in ~70%; perivascular distribution. Associations: Sjögren, HIV/AIDS. |
| LAM | Diffuse bilateral; uniform | Numerous thin-walled round cysts; no zonal predominance | Women of childbearing age. Uniform cysts (vs LCH: upper-lobe, irregular). Complications: PTX, chylous effusion. |
| LCH | Upper/mid > lower; spares costophrenic angles | Nodules (early) → irregular/bizarre cysts (late) | Smoker (95%). Mixed nodule-cyst pattern. Spares bases. |
| PAP | Bilateral; perihilar/central | Crazy paving — GGO + smooth interlobular septal thickening | Classic for PAP but NOT pathognomonic (also PCP, ARDS, COVID-19, edema). Geographic lucent rims. |
UIP vs NSIP — Key Differentiating Features
| Feature | UIP (IPF) | NSIP |
|---|---|---|
| Distribution | Basal, subpleural, posterior; apicobasal gradient | Basal, subpleural — but WITH subpleural sparing |
| Dominant finding | Honeycombing + traction bronchiectasis | Bilateral GGO + reticulation + traction bronchiectasis |
| Honeycombing | Present (hallmark; subpleural stacked cysts 3–10 mm) | Absent or rare (if present: fibrotic NSIP) |
| GGO extent | Minimal; less than fibrosis | Prominent; GGO > reticulation in cellular NSIP |
| Subpleural sparing | Absent — subpleural predominance | Present — key distinguishing feature |
| Temporal pattern | Temporally and spatially heterogeneous | Temporally homogeneous |
| Associations | Idiopathic (IPF); RA; asbestosis; chronic HP | CTD (SSc, PM/DM, Sjögren); drug toxicity; idiopathic |
| Prognosis | Poor; median survival ~3–5 yr | Better; cellular NSIP 5-yr survival ~90%; fibrotic ~60% |
Diagnostic UIP pattern (Fleischner 2018 / ATS 2018): Honeycombing ± peripheral traction bronchiectasis in basal-predominant subpleural distribution — surgical biopsy not required. "Probable UIP": traction bronchiectasis without honeycombing. Features against UIP: upper/mid predominance, peribronchovascular predominance, diffuse bilateral GGO > fibrosis, profuse cysts, air trapping, consolidation in bronchopulmonary segments.
References
Ryerson CJ et al. Diagnosing idiopathic interstitial pneumonias and other ILDs. Eur Respir J. 2025.
Elicker BM, Webb WR. Fundamentals of High-Resolution Lung CT. Lippincott Williams & Wilkins.