What high‑resolution computed tomography (HRCT) thorax findings differentiate viral pneumonia from bacterial pneumonia and fungal pneumonia?

HRCT Differentiation of Viral, Bacterial, and Fungal Pneumonia

Viral pneumonia characteristically presents with bilateral ground-glass opacities (GGOs) and centrilobular nodules without consolidation, bacterial pneumonia shows consolidation with bronchial wall thickening, and fungal pneumonia demonstrates nodules (particularly large nodules >1cm) with the halo sign. 1, 2, 3

Viral Pneumonia HRCT Features

Primary Patterns

• Bilateral ground-glass opacities are the hallmark finding, present diffusely and symmetrically without associated consolidation 1
• Centrilobular nodules appear in 78% of cases, representing bronchiolar inflammation 1
• Consolidation is characteristically absent in viral pneumonia, distinguishing it from bacterial and fungal etiologies 1
• Multiple patchy ground-glass densities with "paving stone-like" appearance (ground-glass with interlobular septal thickening) are typical, particularly in COVID-19 4

Distribution Characteristics

• Subpleural and peribronchovascular predominance with bilateral involvement 4
• Often affects multiple lobes with patchy distribution rather than lobar consolidation 4

Key Distinguishing Features

• Mosaic attenuation pattern may be present, particularly in Pneumocystis jirovecii pneumonia (PCP), which shows extensive symmetric bilateral GGOs in 95% of cases 1, 2
• Absence of nodules helps differentiate PCP from fungal infections (odds ratio 6.834 for PCP when nodules are absent) 2
• Pleural effusion and mediastinal lymphadenopathy are rare in viral pneumonia 4

Bacterial Pneumonia HRCT Features

Primary Patterns

• Air-space consolidation is the dominant finding, present in 85% of bacterial pneumonia cases 1
• Bronchial wall thickening is a significant independent indicator (odds ratio 2.341), more common than in other pneumonia types 2
• Consolidation may show air-bronchogram sign 4

Distribution Characteristics

• Can be lobar, segmental, or patchy in distribution 1
• No specific lobar predilection, though lower lobes are commonly affected 1

Key Distinguishing Features

• Centrilobular nodules are uncommon (present in only 17% of bacterial pneumonia vs. 96% in Mycoplasma and 78% in viral) 1
• Ground-glass opacities, when present, are typically associated with consolidation rather than isolated 1
• Tree-in-bud opacities indicate bronchiolar inflammation with mucoid impaction, suggesting bacterial bronchopneumonia 5

Fungal Pneumonia HRCT Features

Primary Patterns

• Large nodules (>1cm) are the most characteristic finding, present in 54% of fungal pneumonia cases 3
• The halo sign (nodule surrounded by ground-glass opacity) is highly suggestive, seen in 54% of fungal infections vs. 8-9% in bacterial/viral pneumonia 3
• Consolidation occurs in 75% of cases, similar to bacterial pneumonia 1
• Centrilobular nodules are very common (92% of cases), indicating bronchiolar involvement 1

Distribution Characteristics

• Nodules may be randomly distributed or show peribronchovascular predominance 3
• Can be unilateral or bilateral 3

Key Distinguishing Features

• The presence of nodules is a significant independent indicator for fungal infection (odds ratio 2.531) 2
• Invasive aspergillosis shows bronchoinvasive forms with tree-in-bud appearance, centrilobular nodules, and peribronchial consolidation 5
• The combination of large nodules with halo sign is most suggestive of fungal infection, particularly in immunocompromised patients 3

Critical Diagnostic Pitfalls

Overlapping Features

• Consolidation, ground-glass attenuation, and small nodules show significant overlap among all three pneumonia types and cannot reliably differentiate etiology 1, 3
• Mycoplasma pneumoniae pneumonia presents with features intermediate between viral and bacterial patterns (79% consolidation, 96% centrilobular nodules) 1

Special Populations

• In immunocompromised patients, bronchial wall thickening favors bacterial pneumonia, mosaic pattern without nodules favors PCP, and nodules favor fungal infection 2
• Tree-in-bud opacities in upper lobes warrant immediate exclusion of tuberculosis regardless of risk factors 5

Clinical Integration Required

• HRCT alone has limited value for definitive differentiation except for PCP (which has characteristic extensive bilateral symmetric GGOs) and fungal pneumonia with halo sign 1, 2
• Bronchoscopy with bronchoalveolar lavage should be performed within 24-48 hours in immunocompromised or severely ill patients when HRCT is non-diagnostic 5
• Microbiologic confirmation remains essential, as imaging findings consistently overlap 1, 6

Practical Diagnostic Algorithm

1. Assess for consolidation: Present → likely bacterial or fungal; Absent → likely viral or PCP 1
2. Evaluate nodule pattern: Large nodules with halo sign → fungal; Centrilobular nodules without consolidation → viral or Mycoplasma 1, 2, 3
3. Check for bronchial wall thickening: Present → favors bacterial pneumonia 2
4. Assess GGO distribution: Extensive symmetric bilateral GGOs without nodules → PCP 1, 2
5. Look for tree-in-bud pattern: Present → bacterial bronchopneumonia or tuberculosis (especially if upper lobe) 5