High-resolution CT patterns: ground glass, crazy paving, tree-in-bud
- Finding
High-resolution CT uses very thin slices of the lungs; a few named patterns, like hazy glass or a budding tree, point to the cause.
What it is
High-resolution CT (HRCT) images the lungs in very thin slices with a sharp reconstruction filter and a lung window, so the secondary pulmonary lobule becomes visible. This lobule is the smallest unit outlined by septa (about 1 to 2.5 cm across): a small artery and bronchiole run in its centre, and veins and lymphatics run in the septa at its edges. Each disease fills or thickens a different part of it, and that is what the patterns show. Ground glass is a hazy rise in density through which vessels are still seen; consolidation hides them. Crazy paving is ground glass with thickened septa on top, like irregular paving stones. Tree-in-bud is small nodules in the centre of the lobules joined by branching lines, from bronchioles plugged with pus, mucus or fluid. Honeycombing and traction bronchiectasis mean fibrosis. Where the nodules lie (centrilobular, along the lymphatics, or random) and where the disease sits (upper or lower, central or peripheral) narrow the differential further. Extra prone images remove the normal density of the dependent lung, and expiratory images show air trapping.
What it looks like
- Ground glass: hazy increased density with vessels and bronchial walls still visible.
- Consolidation: density high enough to hide the vessels, often with air bronchograms.
- Crazy paving: ground glass plus thickened septa forming polygons, like irregular paving stones.
- Tree-in-bud: centrilobular nodules joined by branching lines, a few millimetres from the pleura.
- Honeycombing: stacked rows of small cysts under the pleura with traction bronchiectasis; basal and subpleural in the UIP pattern.
- Miliary: countless tiny nodules of similar size spread randomly, touching the pleura and fissures.
Why it looks that way
- Airway (centrilobular) disease fills the bronchiole in the lobule's centre: tree-in-bud and centrilobular nodules that spare the pleura.
- Lymphatic (perilymphatic) disease follows the septa, fissures and bronchovascular bundles: sarcoidosis, lymphangitic tumour spread.
- Blood-borne disease seeds evenly and at random: miliary tuberculosis, haematogenous metastases, fungi.
- Ground glass means partial filling of air spaces or thickening of the alveolar walls below the resolution of CT.
| Pattern | What you see | Think of |
|---|---|---|
| Ground glass | Hazy density, vessels still visible | Oedema, infection (pneumocystis, viral), haemorrhage, early ILD |
| Crazy paving | Ground glass with thickened septa | Alveolar proteinosis, oedema, pneumocystis, haemorrhage, ARDS |
| Tree-in-bud | Centrilobular nodules with branching lines | Infectious bronchiolitis, tuberculosis, mycobacteria, aspiration |
| Honeycombing (UIP) | Subpleural stacked cysts, basal, traction bronchiectasis | Idiopathic pulmonary fibrosis, connective tissue disease |
| Perilymphatic nodules | Nodules along fissures, septa and bronchovascular bundles | Sarcoidosis, lymphangitic tumour spread |
| Random (miliary) nodules | Countless tiny even nodules, touching the pleura | Miliary tuberculosis, metastases, fungi |
Differential and pitfalls
- Ground glass: acute oedema, infection (viral, pneumocystis), haemorrhage, early inflammatory or fibrotic disease, hypersensitivity pneumonitis.
- Crazy paving: classically pulmonary alveolar proteinosis, but more often oedema, pneumocystis or other infection, haemorrhage or ARDS.
- Tree-in-bud: infectious bronchiolitis, endobronchial spread of tuberculosis, non-tuberculous mycobacteria, aspiration.
- UIP pattern (basal, subpleural honeycombing with traction bronchiectasis and little ground glass): idiopathic pulmonary fibrosis.
- Pitfall: normal density in the dependent lung or a poor breath-hold looks like ground glass; prone and full-inspiration images settle it.
- Pitfall: tree-in-bud is not specific for tuberculosis; cavities, upper-lobe location and clinical context make tuberculosis likely.
Next step
- Immunosuppressed patient with diffuse ground glass: test for pneumocystis and viruses; bronchoscopy with lavage if sputum is not diagnostic.
- Tree-in-bud with cavities or upper-lobe disease: sputum for acid-fast bacilli, molecular test and culture; isolate while pending.
- Miliary pattern: tuberculosis work-up (sputum, and samples from other sites as advised) and airborne isolation.
- UIP or other fibrotic pattern: pulmonary function tests and referral for multidisciplinary discussion before calling it idiopathic.
- Persistent ground glass nodule or unexplained pattern: ask the radiologist about follow-up CT timing.
Test yourself
Think of the answer, then tap the question to see it.
What separates ground glass from consolidation on CT?
In ground glass the vessels and bronchial walls are still visible through the haze; consolidation is dense enough to hide them.
What does a tree-in-bud pattern represent?
Small bronchioles filled with pus, mucus or fluid, seen as centrilobular nodules joined by branching lines: usually infection or aspiration.
Which disease is the classic cause of crazy paving?
Pulmonary alveolar proteinosis, although oedema, pneumocystis and other infections, haemorrhage and ARDS are more common causes.
What does a UIP pattern on HRCT look like?
Basal, subpleural reticulation with honeycombing and traction bronchiectasis and little ground glass; it is the pattern of idiopathic pulmonary fibrosis.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Interstitial Lung Disease (StatPearls (NCBI Bookshelf), 2026); Idiopathic Pulmonary Fibrosis (StatPearls (NCBI Bookshelf), 2024); Miliary Tuberculosis (StatPearls (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.