# Community-Acquired Pneumonia: Imaging Patterns and Mimics

## Imaging Patterns of Pneumonia

### Lobar (Airspace) Pneumonia

Lobar pneumonia presents as homogeneous consolidation involving one or more segments, often filling an entire lobe. Air bronchograms, where air-filled bronchi remain visible within the surrounding opacified parenchyma, are the hallmark of airspace disease and confirm that the opacity is within the lung parenchyma rather than the pleural space. The consolidation may be non-segmental or segmental and typically respects fissural boundaries. The classic organism is Streptococcus pneumoniae, which produces the prototypical lobar consolidation pattern. On CT, the consolidation appears dense with air bronchograms and may show the CT angiogram sign, where contrast-enhancing pulmonary vessels are visible coursing through the opacified lung. Klebsiella pneumoniae also causes lobar consolidation but is notable for producing an edematous, swollen lobe that may cause the fissure to bulge outward, the so-called bulging fissure sign.

### Bronchopneumonia (Lobular Pneumonia)

Bronchopneumonia has a distinctly different pattern: patchy, multifocal, bilateral airspace opacities centered on the airways. Rather than filling a lobe uniformly, the infection spreads along the bronchial tree, producing a peribronchial and peribronchiolar distribution that tends to involve multiple lobes. On CT, this manifests as centrilobular nodules, tree-in-bud opacities, patchy consolidation, and bronchial wall thickening. Classic organisms include Staphylococcus aureus, Haemophilus influenzae, gram-negative bacteria, and Pseudomonas. The patchy opacities may progress to confluent consolidation. Staphylococcal pneumonia deserves special mention for its tendency to cause cavitation, pneumatoceles (thin-walled air-filled spaces), empyema, and bronchopleural fistula.

### Interstitial (Atypical) Pneumonia

Interstitial pneumonia is characterized by bilateral, diffuse or patchy ground-glass opacities and reticular or reticulonodular opacities, often with relatively preserved lung architecture despite significant clinical symptoms. On CT, findings include ground-glass opacities, interlobular septal thickening, and peribronchial thickening. Classic causative organisms include Mycoplasma pneumoniae, Chlamydophila pneumoniae, and various viruses including influenza, RSV, SARS-CoV-2, and adenovirus. Viral pneumonias typically produce bilateral, lower lobe predominant ground-glass opacities. COVID-19 characteristically shows peripheral and posterior predominant ground-glass opacities with or without consolidation, which may evolve into organizing pneumonia patterns over time.

### Necrotizing Pneumonia and Lung Abscess

Necrotizing pneumonia presents as multiple small cavities within a consolidated area, reflecting destruction of the normal pulmonary architecture. A lung abscess is a more focal process: a thick-walled cavitary lesion, usually greater than 2 cm in diameter, containing an air-fluid level. Abscesses are most common in dependent lung segments (posterior segments of the upper lobes and superior segments of the lower lobes) because aspiration, the most common underlying mechanism, preferentially affects these segments. The typical causative organisms are anaerobes, Staphylococcus aureus, and Klebsiella. An important distinction is between a lung abscess and an empyema with bronchopleural fistula: an abscess forms an acute angle with the pleura and lies within the lung parenchyma, while an empyema forms an obtuse angle and demonstrates the split pleura sign on contrast-enhanced CT.

## Pneumonia Imaging Pattern Summary

| Pattern | Distribution | Radiograph | CT Features | Classic Organisms |
|---------|-------------|------------|-------------|-------------------|
| Lobar (Airspace) | Segmental/lobar, respects fissures | Homogeneous consolidation with air bronchograms | Dense consolidation, CT angiogram sign | S. pneumoniae, Klebsiella |
| Bronchopneumonia | Patchy, multifocal, bilateral, peribronchial | Patchy airspace opacities | Centrilobular nodules, tree-in-bud, bronchial wall thickening | S. aureus, H. influenzae, Pseudomonas |
| Interstitial (Atypical) | Bilateral, diffuse or patchy | GGO, reticular/reticulonodular opacities | GGO, septal thickening, peribronchial thickening | Mycoplasma, viruses (influenza, COVID-19) |
| Necrotizing / Abscess | Dependent segments | Thick-walled cavity with air-fluid level | Cavitation within consolidation, abscess >2 cm | Anaerobes, S. aureus, Klebsiella |

## Organism-Specific Imaging Associations

### Bacterial

Streptococcus pneumoniae classically produces lobar consolidation, round pneumonia (especially in children), and parapneumonic effusion. Klebsiella has an upper lobe predilection, produces the bulging fissure sign, and is associated with rapid cavitation. Staphylococcus aureus causes bilateral patchy consolidation with cavitation, pneumatoceles, and empyema; hematogenous spread from an extrapulmonary source produces septic emboli, seen as multiple peripheral nodules with a feeding vessel sign. Legionella causes rapidly progressive consolidation that is often initially unilateral but becomes bilateral, and may be accompanied by relative bradycardia clinically. Tuberculosis has two distinct imaging presentations depending on the stage of disease: primary TB manifests as middle or lower lobe consolidation with hilar lymphadenopathy, while reactivation TB produces upper lobe or superior segment lower lobe cavitary consolidation, tree-in-bud nodules from endobronchial spread, and occasionally the miliary pattern of diffuse tiny nodules from hematogenous dissemination.

### Viral

Influenza causes bilateral ground-glass opacities and consolidation and may progress to ARDS. COVID-19 characteristically produces bilateral, peripheral, posterior predominant ground-glass opacities, a crazy paving pattern, and consolidation in later stages, with subpleural sparing reported in some cases. The ground-glass opacities may persist as an organizing pneumonia pattern. Varicella (VZV) pneumonia presents as multiple small (2 to 5 mm) nodules that may calcify after the infection resolves, leaving a characteristic pattern of scattered tiny calcifications on subsequent imaging.

### Fungal

Histoplasmosis and coccidioidomycosis produce consolidation with hilar and mediastinal lymphadenopathy, a pattern that can mimic sarcoidosis or lymphoma, and may calcify over time. Aspergillus produces different patterns depending on the host immune status: the angioinvasive form in immunocompromised patients shows the halo sign (ground-glass opacity surrounding a nodule, representing hemorrhage) followed later by the air crescent sign; a semi-invasive form occurs in patients with COPD; and aspergilloma (mycetoma) develops as a fungal ball within a pre-existing cavity. Pneumocystis jirovecii (PJP) produces bilateral perihilar ground-glass opacities with upper lobe predominance and pneumatoceles, characteristically occurring in HIV/AIDS patients with CD4 counts below 200.

## Pneumonia Mimics

### Pulmonary Hemorrhage

Diffuse alveolar hemorrhage presents as bilateral ground-glass opacities and consolidation that can be indistinguishable from pneumonia on imaging alone. The clinical context provides the key clues: underlying vasculitis, anticoagulation therapy, hemoptysis, rapid development of opacities, and gravity-dependent distribution all favor hemorrhage. Causes include Goodpasture syndrome, granulomatosis with polyangiitis, coagulopathy, and mitral stenosis.

### Organizing Pneumonia (Cryptogenic Organizing Pneumonia / COP)

Previously known as bronchiolitis obliterans organizing pneumonia (BOOP), organizing pneumonia presents as peripheral, patchy, and often migratory consolidation that may be bilateral. The reverse halo sign (atoll sign), where a ring of consolidation surrounds a central area of ground-glass opacity, is relatively specific for this diagnosis. Organizing pneumonia does not respond to antibiotics but responds to corticosteroids, and it must be distinguished from infection and lymphoma.

### Eosinophilic Lung Disease

Chronic eosinophilic pneumonia produces peripheral predominant consolidation, classically described as the "photographic negative of pulmonary edema" because the consolidation occupies the lung periphery rather than the central distribution typical of edema. It tends to affect the upper lobes, may be migratory, is associated with peripheral blood eosinophilia, and responds rapidly to corticosteroids. Acute eosinophilic pneumonia produces bilateral ground-glass opacities and consolidation with pleural effusions, resembling ARDS.

### Pulmonary Alveolar Proteinosis

Pulmonary alveolar proteinosis produces bilateral geographic ground-glass opacities with superimposed interlobular septal thickening, creating the "crazy paving" pattern. It can mimic bilateral pneumonia or pulmonary edema but is distinguished by its chronicity and the absence of fever.

### Bronchoalveolar Carcinoma (Adenocarcinoma In Situ / Lepidic Pattern)

Persistent consolidation that does not resolve with antibiotics should raise concern for adenocarcinoma with a lepidic growth pattern. This malignancy may contain air bronchograms and the CT angiogram sign, closely mimicking infectious pneumonia. It can be unifocal or multifocal and may produce mucin, resulting in low-attenuation consolidation. Any non-resolving pneumonia warrants consideration of this diagnosis.

### Pulmonary Infarction

Pulmonary infarction produces a peripheral, pleura-based, wedge-shaped consolidation known as the Hampton hump, with the apex of the wedge pointing toward the hilum. It may cavitate and is most common in the lower lobes. Associated pulmonary embolism should be sought on CT pulmonary angiography.

## Complicated Pneumonia

### Parapneumonic Effusion and Empyema

Parapneumonic effusions exist on a spectrum. Simple parapneumonic effusions are free-flowing, appear anechoic on ultrasound, and resolve with antibiotic treatment alone. Complicated parapneumonic effusions are septated with echogenic fluid and may require drainage. Empyema represents frank pus in the pleural space. On contrast-enhanced CT, empyema demonstrates the split pleura sign: enhancing, thickened visceral and parietal pleura separated by the infected fluid collection. The collection has a lenticular shape conforming to the chest wall and forms obtuse angles with the lung.

### Parapneumonic Effusion vs. Lung Abscess on CT

The distinction between empyema and lung abscess has important management implications. Empyema forms obtuse angles with the chest wall, shows the split pleura sign, compresses the adjacent lung without destroying it, and has a lenticular shape. A lung abscess forms acute angles with the chest wall, has an irregular thick wall, destroys the surrounding lung parenchyma, and has a round shape.

| Feature | Empyema | Lung Abscess |
|---------|---------|-------------|
| Shape | Lenticular | Round |
| Angle with chest wall | Obtuse | Acute |
| Split pleura sign | Present | Absent |
| Wall | Smooth, enhancing pleura | Irregular, thick |
| Adjacent lung | Compressed but intact | Destroyed |
| Management | Drainage (chest tube/surgery) | Antibiotics ± percutaneous drainage |

## Imaging Follow-Up

### Resolution Patterns

Most bacterial pneumonias show radiographic improvement within 2 to 4 weeks, though complete radiographic resolution may take 6 to 12 weeks, particularly in older patients and those with COPD. The ACR and ATS recommend a follow-up chest radiograph at 6 to 12 weeks for patients over 50 or those with risk factors for malignancy to confirm resolution and exclude an underlying mass. Non-resolving pneumonia, defined as a persistent opacity after 12 weeks of appropriate therapy, requires further evaluation with contrast-enhanced chest CT, bronchoscopy, or biopsy to exclude malignancy or another mimic.

<image>A four-panel diagram comparing the three main imaging patterns of pneumonia on chest radiograph and CT. Panel 1 shows lobar pneumonia with homogeneous consolidation filling the right upper lobe with visible air bronchograms and the fissure as a sharp inferior boundary. Panel 2 shows bronchopneumonia with patchy, bilateral, multifocal opacities centered on the airways, with corresponding CT showing tree-in-bud nodules and peribronchial thickening. Panel 3 shows interstitial (atypical) pneumonia with bilateral reticular and ground-glass opacities without dense consolidation, with corresponding CT showing diffuse ground-glass opacity. Panel 4 shows a lung abscess as a thick-walled cavitary lesion with an air-fluid level in the right lower lobe with surrounding consolidation.</image>

<image>A CT image diagram showing the split pleura sign differentiating empyema from lung abscess. On the left, an empyema is depicted as a lenticular fluid collection making an obtuse angle with the chest wall, with enhancing thickened visceral and parietal pleura separated by fluid (split pleura sign labeled). The adjacent lung is compressed but architecturally normal. On the right, a lung abscess is shown as a round, thick-walled cavity making an acute angle with the chest wall, with irregular destruction of the adjacent lung parenchyma and no split pleura sign. Key differentiating features are listed in a comparison table below the images.</image>

## Clinical Pearls

Air bronchograms within a consolidation confirm an airspace process and help localize the opacity to the lung parenchyma rather than the pleural space. Non-resolving pneumonia in a patient over 50 must be followed to resolution with imaging, because persistent consolidation raises concern for underlying malignancy, particularly lepidic-predominant adenocarcinoma. The reverse halo sign (atoll sign) on CT is relatively specific for organizing pneumonia and should prompt consideration of this diagnosis when consolidation does not respond to antibiotics. Round pneumonia is a well-recognized pattern in children, especially under age 8, and should not be confused with a mass; in adults it is less common but can occur and should be followed to resolution. When differentiating empyema from lung abscess, the angle with the chest wall (obtuse versus acute) and the split pleura sign on contrast-enhanced CT are the key distinguishing features. In immunocompromised patients, the differential for pulmonary opacities expands dramatically to include PJP, fungal infection, viral reactivation (CMV), and drug toxicity alongside typical bacterial pneumonia.

## References

- Franquet T. "Imaging of Pulmonary Viral Pneumonia." *Radiology*, 2011
- Kligerman SJ, et al. "From the Radiologic Pathology Archives: Organization and Fibrosis as a Response to Lung Injury." *RadioGraphics*, 2013
- Reynolds JH, et al. "Pneumonia in the Immunocompetent Patient." *British Journal of Radiology*, 2010
- Metlay JP, et al. "Diagnosis and Treatment of Adults with Community-Acquired Pneumonia: An Official ATS/IDSA Clinical Practice Guideline." *American Journal of Respiratory and Critical Care Medicine*, 2019
- Walker CM, et al. "Imaging Pulmonary Infection: Classic Signs and Patterns." *AJR*, 2014