Diagnostic imaging of pneumonias

The role of radiography and computed tomography

Chest radiography is the primary and main imaging modality when pneumonia is suspected. Pulmonary opacities usually become visible within 12 hours of symptom onset. The main goal of a chest X-ray is not to search for a specific microbe (specific patterns do not exist), but to assess the extent of the process and identify complications (cavitation, pleural effusion).

High-resolution computed tomography (HRCT) uses slice thicknesses of less than 2 mm and significantly outperforms radiography in sensitivity. Indications for HRCT include:

• Clinical suspicion of infection with a clear chest X-ray;
• High risk of pulmonary infections (e.g., in neutropenia);
• Persistent pulmonary opacities (to rule out alternative pathology).

Complete resolution of pneumonia on imaging takes anywhere from 1–2 weeks to 2 months, depending on the pathogen and age.

Main patterns of pulmonary infection

Based on the imaging appearance, pneumonias are traditionally divided into three types:

1. Lobar pneumonia (Lobar pneumonia). The exudate spreads via collateral pathways (pores of Kohn). Imaging shows a uniform, homogeneous opacification of a segment or lobe with the air bronchogram sign. A rare variant is round pneumonia, which mimics a mass lesion. The most frequent pathogen is Streptococcus pneumoniae.
2. Bronchopneumonia (lobular pneumonia). Initially focal in nature with peribronchiolar inflammation. The air bronchogram sign is absent. HRCT reveals centrilobular nodules and branching linear opacities (the "tree-in-bud" sign — tree-in-bud). Typical for Staphylococcus aureus and gram-negative bacilli.
3. Interstitial pneumonia (atypical). Manifests as focal or diffuse small, heterogeneous opacities. Typical for viral infections and Mycoplasma pneumoniae. Notably, in 10% of patients with confirmed Pneumocystis pneumonia, the chest X-ray may be completely normal.

Imaging of pneumonia complications

Pulmonary infections can lead to a number of structural changes that require imaging follow-up:

• Lung abscess: a localized necrotic cavity containing pus, most commonly occurring in gravitationally dependent areas due to aspiration of anaerobes.
• Pneumatocele: a thin-walled air-filled cavity resulting from check-valve bronchial obstruction (typical for S. aureus in children or Pneumocystis in AIDS).
• Empyema thoracis: an encapsulated infected pleural effusion. CT typically shows the "split pleura sign" (split pleura sign) — contrast enhancement of the thickened parietal and visceral pleural layers.
• Septic emboli: multiple cavitating nodules. On transverse CT slices, a blood vessel can often be seen leading to the nodule (the "feeding vessel sign" — feeding vessel sign).
• Pulmonary gangrene: necrosis with the formation of a pulmonary sequestrum inside a cavity. Often associated with bulging of the interlobar fissure (the "bulging fissure sign" — bulging fissure sign), which is particularly characteristic of Klebsiella pneumoniae.

Class and exam questions

What else to study on this topic

• Clinical and imaging features of hospital-acquired (HAP) and ventilator-associated (VAP) pneumonia
• Specifics of pneumonia in patients with AIDS (Pneumocystis jiroveci)
• Differential diagnosis of cavitating pneumonia in immunocompetent patients
• Pathogenesis and CT signs of a bronchopleural fistula
• Features of pulmonary infections in the setting of severe emphysema and interstitial lung disease

Other topics in the «Chest Radiology» section

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