A 42-year-old woman with known pulmonary tuberculosis undergoes high-resolution CT chest. Multiple imaging patterns are identified. Which is the most common radiological pattern seen in active pulmonary tuberculosis?

Explanation

## Most Common Radiological Pattern in Active TB on HRCT **Key Point:** Tree-in-bud opacities with bronchial wall thickening are the most common and characteristic HRCT pattern of **active** pulmonary tuberculosis, reflecting endobronchial spread of infection. ### Radiological Patterns of TB — Frequency on HRCT | Pattern | Frequency | Clinical Significance | Typical Location | |---------|-----------|----------------------|------------------| | **Tree-in-bud opacities + bronchial wall thickening** | Most common (~60–80%) | Endobronchial/bronchogenic spread; active disease | Upper lobe, centrilobular | | Consolidation ± cavitation | Common (~40–60%) | Active disease; highly infectious | Upper lobe apical-posterior | | Solitary nodule ± halo | Less common (~10–15%) | Early/nodular TB | Variable | | Bronchiectasis | Sequela (~5–10%) | Chronic/healed TB | Lower lobes | **High-Yield:** On **HRCT**, tree-in-bud opacities represent: - Centrilobular nodules (2–4 mm) connected by branching linear structures - Filling of terminal and respiratory bronchioles with caseous material, mucus, or pus - Pathognomonic of **bronchogenic spread** of TB — the dominant mechanism of active disease dissemination ### Mechanism of Tree-in-Bud Pattern in TB 1. **Caseous material** from a primary focus liquefies and drains into the bronchial tree 2. **Bronchogenic spread** seeds distal airways (terminal/respiratory bronchioles) 3. **Centrilobular nodules** form as bronchioles fill with infected material 4. **Branching pattern** on HRCT resembles a "tree in bud" (budding tree in spring) 5. **Bronchial wall thickening** accompanies the peribronchial inflammation **Clinical Pearl:** The question specifically asks about HRCT findings in **active** TB. While cavitation is a well-known feature, tree-in-bud opacities are the most frequently identified pattern on HRCT in active disease, as described in standard radiology references (Webb, Müller & Naidich — *High-Resolution CT of the Lung*; Hansell et al.). Cavitary lesions, though highly specific, are present in a smaller proportion of HRCT-confirmed active TB cases. ### Why Other Options Are Less Correct - **Cavitary lesion with surrounding consolidation** (Option C): Highly specific for TB and indicates active disease, but cavitation is seen in only ~40–50% of active TB cases on HRCT — less frequent than tree-in-bud opacities - **Solitary nodule with halo sign** (Option B): Halo sign is more characteristic of angioinvasive aspergillosis; uncommon in TB - **Bronchiectasis with bronchial dilatation** (Option D): A sequela of chronic/healed TB, not a primary active disease pattern **Mnemonic:** **TIB** = **T**B → **I**nfected bronchioles → **B**ronchogenic spread = Tree-In-Bud *Reference: Webb, Müller & Naidich, High-Resolution CT of the Lung, 5th ed.; Hansell DM et al., Radiology 2008 (Fleischner Society Glossary)*