Definitions:
- Ground glass opacity (GGO): Hazy increased opacity with preservation of bronchial and vascular margins. Differentiated from consolidation, which obliterates vessels and airway margins.
- Reticular opacity: Mesh-like network of fine lines; may represent interlobular septal thickening, peribronchiovascular thickening, traction bronchiectasis, or honeycombing.
- ILA (Interstitial Lung Abnormality): Reticular opacities >5% of the lungs; reported in 7–11% of elderly asymptomatic patients; associated with increased mortality risk.
Ground Glass Opacities (Fig 5)
| Pattern | Finding | Diagnosis to Consider | Action |
|---|---|---|---|
| Basilar — atelectasis/low lung volumes | — | Dependent atelectasis | No further workup |
| Acute (by comparison imaging or history) | Diffuse or focal | Infectious, inflammatory, edema | Workup based on clinical scenario |
| Chronic — diffuse | Air trapping on expiratory imaging | Hypersensitivity Pneumonitis | Pulmonary consultation may be indicated |
| Chronic — diffuse | Septal thickening ("crazy paving") | Alveolar Proteinosis | Pulmonary consultation may be indicated |
| Chronic — focal, nodular | — | Adenocarcinoma | Pulmonary consultation may be indicated |
| Chronic — lower lung / subpleural | Reticular fibrotic opacities ± traction bronchiectasis | NSIP or DIP | Pulmonary consultation may be indicated |
| Chronic — lower lung / subpleural | Associated consolidation | Organizing Pneumonia | Pulmonary consultation may be indicated |
Reticular Opacities / Interlobular Septal Thickening (Fig 6)
Excludes reticular opacities secondary to atelectasis or low lung volumes.
| Pattern | Subtype | Diagnosis to Consider | Action |
|---|---|---|---|
| Scattered / random | — | Non-specific / benign | No further workup |
| Interlobular septal thickening | Smooth | Pulmonary edema | Clinical correlation |
| Interlobular septal thickening | Nodular | Lymphangitic carcinomatosis | Further workup |
| Subpleural, basilar | No additional findings; <5% of lungs | Probable non-significant ILA | No further workup |
| Subpleural, basilar | Mild (>5%) or additional findings (diffuse nodules, GGO, cysts) | ILA | Thin-section CT with expiratory and prone imaging |
| Subpleural, basilar | Traction bronchiectasis + honeycombing | UIP (usual interstitial pneumonia) | Pulmonary consultation may be indicated |
| Subpleural, basilar | Traction bronchiectasis; no honeycombing | NSIP (nonspecific interstitial pneumonia) | Pulmonary consultation may be indicated |
ILA surveillance: Reticular opacities >5% of the lungs are associated with increased all-cause mortality and postoperative complications. If patients become symptomatic, follow-up imaging or consultation is warranted. Thin-section CT with expiratory and prone sequences helps differentiate true fibrosis from gravity-dependent changes.
UIP vs. NSIP: UIP = peripheral, subpleural, basilar honeycombing ± traction bronchiectasis. NSIP = subpleural sparing may be seen; traction bronchiectasis present; no honeycombing. The Fleischner Society classification schema (typical UIP, probable UIP, indeterminate, alternate diagnosis) applies for suspected IPF.
Reporting Elements
For GGO:
- Acute vs. chronic (comparison imaging or clinical history)
- Diffuse vs. focal
- Distribution — upper vs. lower, central vs. subpleural, random
- Associated findings — reticular opacities, cardiomegaly, pleural effusions
For reticular opacities:
- Distribution
- Associated findings (nodules, GGO, traction bronchiectasis, honeycombing)
References
Munden RF, Black WC, Hartman TE, et al. Managing Incidental Findings on Thoracic CT: Lung Findings. A White Paper of the ACR Incidental Findings Committee. J Am Coll Radiol. 2021;18(9):1267–1279.