# HRCT Patterns in Lung Disease

> HRCT patterns in lung disease for NEET-PG Medicine covering ground-glass opacity, honeycombing, tree-in-bud, crazy paving and their diagnostic associations.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/hrct-patterns-in-lung-disease
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Harrison's Principles of Internal Medicine; Murray and Nadel's Textbook of Respiratory Medicine; Fleischner Society glossary of terms for thoracic imaging
- How to cite: "HRCT Patterns in Lung Disease", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/hrct-patterns-in-lung-disease

## Direct answer

HRCT patterns are the high-resolution CT appearances of lung disease, read by pattern, distribution and reversibility rather than by a single finding. Ground-glass opacity is hazy increased attenuation with preserved bronchovascular outlines, and it reflects partly filled or thickened alveolar spaces. Honeycombing marks established fibrosis, most typically in usual interstitial pneumonia. Tree-in-bud indicates bronchiolar disease, and crazy paving combines ground glass with septal thickening.

## What you must remember

- **Ground-glass opacity:** hazy increase in attenuation with preserved bronchial and vascular outlines, seen in infection, alveolar haemorrhage and hypersensitivity pneumonitis.
- **Honeycombing:** clustered cystic air spaces with thick walls, typically basal and subpleural, and it does not reverse with treatment.
- **Tree-in-bud:** centrilobular nodules with branching linear opacities, pointing to bronchiolar spread from infection or aspiration.
- **Crazy paving:** ground glass with superimposed interlobular and intralobular septal thickening, classic for pulmonary alveolar proteinosis.
- **Reversed halo sign:** central ground glass surrounded by a ring of consolidation, classic for cryptogenic organising pneumonia.
- **Centrilobular nodules:** small nodules centred on the bronchioles, typical of hypersensitivity pneumonitis and bronchiolar disease.

## A 40-year-old woman with ground glass that clears

A 40-year-old woman with a dry cough and breathlessness has bilateral ground-glass opacities on HRCT. Her symptoms improve, and a repeat scan four weeks later shows the opacities have partly cleared.

HRCT is read by pattern, distribution and reversibility. Patterns overlap between diseases, so no single finding settles the diagnosis. The table lists the patterns most often tested in exams, and the associations give the starting differential.

| Pattern | Appearance | Classic associations |
|---|---|---|
| Ground-glass opacity | Hazy, vessels preserved | Infection, haemorrhage, NSIP |
| Honeycombing | Cysts with thick walls, basal | UIP, end-stage fibrosis |
| Tree-in-bud | Centrilobular nodules, branching | Infection, aspiration, bronchiectasis |
| Crazy paving | Ground glass with septal thickening | Alveolar proteinosis, Pneumocystis |
| Centrilobular nodules | Small nodules near bronchioles | Hypersensitivity pneumonitis |

Reversibility is the next question, since ground glass that clears suggests an acute or inflammatory process, whereas persistent fibrosis does not. Distribution then narrows the list further, with basal and subpleural change favouring fibrosis and upper-zone, perilymphatic nodules favouring sarcoidosis.

## Where the exam tests pattern recognition

The exam tests whether you name the pattern correctly and avoid calling a reversible finding fibrosis.

- **Trap:** reading ground-glass opacity as fibrosis — **Correct:** ground glass is usually reversible filling or inflammation, while fibrosis shows traction bronchiectasis and honeycombing.
- **Trap:** calling tree-in-bud a tumour sign — **Correct:** it indicates bronchiolar disease, most often infective, so microbiology comes before biopsy.
- **Trap:** calling crazy paving diagnostic of alveolar proteinosis alone — **Correct:** it is typical there, but it also occurs in Pneumocystis pneumonia, pulmonary oedema and haemorrhage.
- **Trap:** naming the disease from a single slice — **Correct:** pattern, distribution and the clinical picture must be combined.

## Frequently asked questions

### What does honeycombing on HRCT indicate?

Honeycombing on HRCT indicates established fibrosis, with clustered cystic air spaces that have thick walls. It is most typical of usual interstitial pneumonia and idiopathic pulmonary fibrosis, and honeycombing does not reverse with treatment.

### What is crazy paving on HRCT?

Crazy paving is ground-glass opacity with superimposed interlobular and intralobular septal thickening. It is classic for pulmonary alveolar proteinosis, but it is also seen in Pneumocystis pneumonia, pulmonary oedema and alveolar haemorrhage.

### Which HRCT pattern suggests bronchiolar disease?

The tree-in-bud pattern, with centrilobular nodules and branching linear opacities, suggests bronchiolar disease. It is most often caused by infection such as tuberculosis or nontuberculous mycobacteria, or by aspiration, so microbiology is the first step.

### What does a reversed halo sign on HRCT suggest?

A reversed halo sign is a central ground-glass area surrounded by a ring of consolidation, and it is classic for cryptogenic organising pneumonia. It can also appear in other conditions, so it supports the diagnosis rather than proving it alone.