Pneumonia Patterns
| Pattern | Typical Organisms | Key CT/CXR Features | Notes |
|---|---|---|---|
| Lobar/segmental consolidation | S. pneumoniae, Klebsiella | Dense homogeneous consolidation within one lobe or segment; air bronchograms; Klebsiella: "bulging fissure" sign | Most common bacterial pattern; Klebsiella classically upper lobe (alcoholics, diabetics) |
| Bronchopneumonia/lobular | Staph aureus, Haemophilus, aspiration | Patchy bilateral airspace opacities; peribronchial distribution; centrilobular nodules; tree-in-bud; lower lobe predominance | Aspiration: posterior upper lobe segments and superior lower lobe segments (supine) |
| Interstitial/atypical | Mycoplasma, viral (influenza, RSV, COVID-19), PCP | Reticular opacities; GGO; bilateral symmetric; peribronchial thickening; mosaic attenuation | PCP: perihilar bilateral GGO in HIV; pneumatoceles → spontaneous PTX risk; LDH elevated |
| Cavitary | Staph aureus, Klebsiella, TB, fungal (Aspergillus, Mucor), anaerobes | Air-filled cavity; wall >4 mm; air-fluid level = abscess; satellite nodules (TB) | TB: upper lobe + apical lower lobe; cavitation + tree-in-bud = active TB until proven otherwise |
| Round pneumonia | S. pneumoniae (children >> adults) | Spherical/ovoid consolidation; posterior lower lobes; well-defined margins; can mimic mass | More common <8 yo; follow to resolution to exclude malignancy in adults |
Complications to report: Lung abscess (thick-walled cavity with air-fluid level) · Empyema (split pleura sign, lenticular collection, pleural enhancement) · Bronchopleural fistula (air-fluid level at pleural margin + pneumothorax component) · Necrotizing pneumonia (GGO → consolidation → cavitation; absent enhancement within necrotic zones)
PCP pearl: Bilateral perihilar GGO in HIV-positive patient with elevated LDH = PCP until proven otherwise. Pneumatoceles develop in ~30% → spontaneous pneumothorax risk. May have near-normal CXR early — HRCT far more sensitive.
CT Pattern Approach — Infectious Differential
| CT Pattern | Common Infectious Causes | Key Differentiating Features |
|---|---|---|
| Lobar/segmental consolidation | S. pneumoniae, Klebsiella, Legionella, Haemophilus | Air bronchograms; bulging fissure sign (Klebsiella); Legionella: lower lobe, rapid progression, may cavitate |
| Bronchopneumonia (patchy, bilateral) | Staph aureus, aspiration organisms, Haemophilus, gram-negatives | Peribronchial distribution; centrilobular nodules; lower lobe; tree-in-bud |
| Ground-glass opacity (GGO) | Viral (influenza, RSV, COVID-19, CMV), PCP, early bacterial | Bilateral symmetric; COVID-19: peripheral + lower lobe + crazy paving; PCP: perihilar; influenza: diffuse |
| Nodular pattern | TB (miliary), fungal (Histoplasma, Coccidioides), septic emboli, Cryptococcus | Miliary (<2 mm, random): TB, fungal · Centrilobular: endobronchial spread · Subpleural: aspergillosis, septic emboli |
| Halo sign | Angioinvasive Aspergillus (neutropenic), Mucor, Candida | GGO halo around nodule = hemorrhage from angioinvasion; CT early before cavitation (air crescent sign) |
| Reversed halo sign (atoll sign) | Mucormycosis, organizing pneumonia, PCP | Ring of consolidation around central GGO; Mucor in immunocompromised — aggressive angioinvasion |
| Cavitation | Staph (pneumatoceles), Klebsiella, TB, anaerobes (lung abscess), fungal | Thick wall + air-fluid level = abscess; thin-walled = pneumatocele; upper lobe + satellite nodules = TB |
| Tree-in-bud | Endobronchial TB, MAC, aspiration, bronchopneumonia | Centrilobular branching opacities; endobronchial spread of infection |
| Crazy paving | PCP, viral pneumonia, PAP, COVID-19 | GGO + interlobular septal thickening; PCP in HIV; consider PAP if non-infectious |
Fungal pearls: Endemic fungi (Histo, Coccidioides, Blasto) based on geography. Opportunistic fungi (Aspergillus, Mucor, Candida) in immunocompromised. Aspergilloma = fungus ball within pre-existing cavity (air crescent sign, Monod sign). Angioinvasive Aspergillus in neutropenia → halo sign early, air crescent sign on recovery.
HIV / Immunocompromised Pulmonary Infections
CD4 count guides the differential:
- >500: bacterial pneumonia, TB
- 200–500: bacterial, TB, early PCP
- 100–200: PCP, MAC, CMV
- <100: PCP, MAC, CMV, disseminated fungal (Cryptococcus, Aspergillus, Histoplasma), Kaposi sarcoma
| Organism | CT Pattern | Key Features |
|---|---|---|
| P. jiroveci (PCP) | Bilateral perihilar / upper lobe GGO; spares costophrenic angles; crazy paving on HRCT; no pleural effusion | CD4 <200; elevated LDH; pneumatoceles (30%) → PTX; CXR near-normal early |
| Bacterial (S. pneumoniae, Pseudomonas) | Lobar or multifocal consolidation | Any CD4; Pseudomonas at low CD4; recurrent bacterial pneumonia (≥2/yr) = AIDS-defining |
| Tuberculosis | CD4 >200: typical (upper lobe, cavitation, tree-in-bud) · CD4 <200: atypical — lower lobe, miliary, mediastinal LAD, no cavitation | Consider TB at any CD4; miliary = hematogenous dissemination; low-density necrotic nodes = TB |
| MAC (M. avium complex) | Tree-in-bud; centrilobular nodules; consolidation; bilateral symmetric | CD4 <50; often disseminated |
| CMV pneumonitis | Bilateral GGO + consolidation; peribronchovascular; small nodules | CD4 <50; coexists with other OIs; treat with ganciclovir |
| Cryptococcus | Single or multiple nodules/masses; consolidation; cavitation; miliary pattern | Any low CD4; often disseminated with meningitis; serum/CSF cryptococcal antigen |
| Kaposi sarcoma | Flame-shaped perihilar consolidation; nodules along bronchovascular bundles; Kerley B lines; pleural effusion; mediastinal LAD | HHV-8 related; skin lesions usually present; diagnosis by bronchoscopy |
Reference
Kanne JP et al. Pulmonary Manifestations of HIV Infection. AJR Am J Roentgenol. 2012;198(6).