A 45-year-old woman with known pulmonary tuberculosis undergoes high-resolution CT chest. Which is the most common radiological pattern seen in active pulmonary tuberculosis?

Explanation

## Most Common Radiological Pattern in Active Pulmonary TB ### Characteristic HRCT Findings in Active TB **Key Point:** Centrilobular nodules with bronchial wall thickening (bronchitis pattern) is the most common HRCT finding in active pulmonary tuberculosis, reflecting endobronchial spread and inflammation. ### Pathophysiology of Centrilobular Nodules Centrilobular nodules occur due to: 1. **Endobronchial spread** — tubercle bacilli spread along bronchi and bronchioles 2. **Bronchial wall inflammation** — active inflammation of small airways 3. **Peribronchial consolidation** — inflammatory exudate around terminal bronchioles These nodules are **1–4 mm in size** and distributed around the centrilobular region (around small airways), NOT at the pleura or septa. ### Radiological Patterns in TB: Frequency Table | Pattern | Frequency | Appearance | Stage | | --- | --- | --- | --- | | Centrilobular nodules + bronchial wall thickening | **40–50%** | **Most common** | Active disease | | Cavitation | 30–40% | Apical, thick-walled | Reactivation TB | | Consolidation | 25–35% | Lobar or segmental | Active/reactivation | | Pleural effusion | 10–20% | Unilateral, apical | Hypersensitivity/active | | Interstitial fibrosis | 5–10% | Apical, chronic | Healed/chronic TB | ### High-Yield HRCT Signs in TB **Mnemonic: CRAB** — **C**entrilobular nodules, **R**ough bronchial walls, **A**pical location, **B**ronchial wall thickening **Clinical Pearl:** Centrilobular nodules are highly specific for endobronchial TB and are seen in the early phase of active disease. Their presence indicates active mycobacterial replication and spread along airways — this is why antituberculous therapy causes them to resolve within 2–3 months. ### Why This Pattern Is Most Common Endobronchial spread is the hallmark of active TB: - Occurs in ~80% of active TB cases - Precedes cavitation - Seen in both primary and reactivation TB - Reflects ongoing bacterial multiplication and airway inflammation ### Why Not Other Patterns? - **Peripheral ground-glass opacities:** Seen in viral pneumonia, atypical pneumonia, or hypersensitivity pneumonitis — NOT typical of TB - **Diffuse interstitial fibrosis:** Pattern of healed/chronic TB or pulmonary fibrosis (IPF), not active disease - **Pleural effusion with lymphadenopathy:** Seen in 10–20% of cases, usually in primary TB or hypersensitivity; NOT the most common pattern