What are the differences between pneumonia and pulmonary infarction on chest X‑ray and chest CT?

Radiographic Differentiation: Pneumonia vs Pulmonary Infarction

Pulmonary infarction appears as peripheral, pleural-based consolidations with central lucencies and absent air bronchograms, while pneumonia typically shows air bronchograms within consolidations that can occur anywhere in the lung.

Key Distinguishing Features on Imaging

Chest X-Ray Findings

Pulmonary Infarction:

• Peripheral, pleural-based opacity with sharp, convex border toward the hilum (semicircular or "cushion-like" appearance) 1
• The classic "triangular" shape is actually uncommon—the apical portion is typically spared due to collateral blood flow 1
• Located along visceral pleura (costal, diaphragmatic, mediastinal, or interlobar surfaces) 1
• May show the Westermark sign (focal hyperlucency) but this is rare 2

Pneumonia:

• Can occur in any lung location (not necessarily peripheral)
• More commonly shows air bronchograms
• Less defined borders, more heterogeneous appearance

CT Chest Findings (Most Discriminatory)

Pulmonary Infarction - Highly Specific Features:

Central lucencies within the consolidation are the single most specific finding (98% specificity, 46% sensitivity) 3. This represents the "reversed halo sign" or sponge-like appearance reflecting dual blood supply with foci of preserved non-infarcted lung adjacent to infarcted areas 2, 4.

Additional CT characteristics include:

• Focal decreased or absent enhancement compared to atelectatic lung (95% of cases) 4
• Vessel sign: enlarged vessel at the apex of consolidation (32% of infarcts vs 11% of other consolidations) 3, 4
• Absence of air bronchograms (only 8% of infarcts vs 40% of other consolidations) 3
• Broad pleural base (65%) with truncated apex (57%) 4
• Convex border toward hilum (46%) 4
• Linear stranding from apex toward hilum (24%) 4
• Heterogeneous enhancement with rim of more defined consolidation 2

Pneumonia:

• Air bronchograms present in 40% of peripheral consolidations 3
• More homogeneous enhancement
• No central lucencies (only 2% of non-infarct consolidations show this) 3
• Can occur centrally or peripherally

Diagnostic Algorithm

When to Suspect Pulmonary Infarction Over Pneumonia:

1. Clinical context matters critically 5:

   • Pleuritic chest pain is significantly more frequent with infarction (p=0.0064) 4
   • Elevated D-dimer levels 5
   • Tachycardia out of proportion to fever 5
   • Risk factors: thrombophilic conditions (RA, malignancy), PE involving >1 lobe, heart failure 4
   • Younger age, tall stature, active smoking predict infarction in PE setting 1

2. Imaging sequence:

   • If chest X-ray shows peripheral, pleural-based consolidation with convex border → high suspicion for infarction
   • Proceed directly to CT pulmonary angiography (CTPA) to identify PE and confirm infarction 2
   • On CTPA, look for the diagnostic triad: central lucencies + vessel sign + absent air bronchograms (99% specificity when combined) 3

3. DECT/Subtraction CT adds value:

   • Pulmonary infarcts show no enhancement on perfusion images 2
   • May show increased enhancement in immediately adjacent normal lung 2

Critical Pitfalls to Avoid

• Don't assume triangular shape is required for infarction—this is a deeply rooted misconception; most infarcts are semicircular or cushion-shaped 1
• Don't mistake infarction for pneumonia in immunocompromised patients—always check D-dimer and consider PE, especially in thrombophilic conditions like RA 5
• Don't rely on chest X-ray alone—CT is far superior for distinguishing these entities 3, 4
• Mosaic attenuation and enlarged bronchial arteries are NOT seen in acute PE alone—their presence suggests chronic thromboembolic disease 2

When Imaging Findings Overlap

If clinical presentation suggests pneumonia but imaging shows peripheral consolidation without clear air bronchograms, measure D-dimer and proceed to CTPA if elevated or if patient has pleuritic pain 5. The presence of central lucencies on CT essentially confirms infarction over pneumonia (positive likelihood ratio of 23.0) 3.