CT Imaging Features of Pneumonia: A Comprehensive Guide to Radiological Patterns

Pneumonia, classified by anatomical distribution, is primarily divided into three types: lobar pneumonia, bronchopneumonia (lobular pneumonia), and interstitial pneumonia. Each type exhibits distinct radiological characteristics on computed tomography (CT) scans, enabling clinicians to assess disease progression, localization, and underlying pathology more accurately. Understanding these imaging patterns is crucial for timely diagnosis and effective treatment planning.

Lobar Pneumonia: Stages and CT Manifestations

Lobar pneumonia typically affects an entire lobe of the lung and progresses through four well-defined pathological stages: congestion, red hepatization, gray hepatization, and resolution. Each stage corresponds to specific changes visible on CT imaging.

1. Congestion Stage

In the early phase of infection, CT scans reveal ground-glass opacities within a specific lung lobe. These hazy areas appear with ill-defined margins, indicating alveolar edema and early inflammatory infiltration. The affected region often shows increased attenuation compared to normal lung tissue but maintains air bronchograms—visible air-filled bronchi surrounded by opaque lung parenchyma—a key diagnostic clue.

2. Red and Gray Hepatization Stages

As the disease advances, the lung tissue becomes increasingly consolidated due to fibrin and cellular exudate filling the alveoli. During both red and gray hepatization, CT demonstrates dense, homogeneous consolidation that conforms to lobar or segmental anatomy. This "hepatized" appearance mimics the texture of the liver, hence the name. Air bronchograms are commonly observed, further supporting the diagnosis of bacterial pneumonia.

3. Resolution (Dissolution) Stage

In the final phase, inflammation begins to subside, and the immune system clears the infectious agents. On follow-up CT scans, the previously dense consolidations gradually decrease in density. They evolve into patchy, irregular areas of residual opacity scattered throughout the affected lobe. Complete resolution may take several weeks, depending on patient health and pathogen virulence.

Bronchopneumonia (Lobular Pneumonia): Focal Infection Patterns

Bronchopneumonia presents as multifocal, patchy infiltrates predominantly located in the middle and lower lung zones. Unlike lobar pneumonia, it follows a peribronchial distribution, often associated with infected small airways. CT imaging typically shows:

• Thickened bronchovascular bundles due to inflammation along the airway walls
• Ill-defined, variable-sized consolidative patches centered around terminal bronchioles
• Patchy ground-glass opacities interspersed with areas of partial collapse

In cases where small airways become obstructed by mucus or inflammatory debris, secondary complications such as lobular hyperinflation (indicative of air trapping) or segmental atelectasis (lung collapse) may develop. These findings highlight the importance of airway-centered inflammation in this form of pneumonia.

Interstitial Pneumonia: Diffuse Lung Involvement

Interstitial pneumonia primarily affects the connective tissue framework of the lungs, including the walls of small bronchioles and the interstitium surrounding alveoli. It is commonly caused by viral infections, atypical bacteria (e.g., Mycoplasma pneumoniae), or autoimmune conditions.

Key CT Findings Include:

Reticular opacities: Fine net-like patterns reflecting thickened interlobular septa and fibrous tissue proliferation.

Peribronchovascular thickening: Increased prominence of central and peripheral bronchial and vascular structures due to inflammatory cell infiltration along their sheaths.

Small patchy ground-glass areas: Irregular hazy regions suggesting mild alveolar involvement alongside interstitial inflammation.

Hilar lymphadenopathy: Enlarged mediastinal or hilar lymph nodes may be present, particularly in infectious or reactive etiologies.

This pattern often appears bilaterally and symmetrically, especially in the lower lobes, and can mimic other diffuse lung diseases such as pulmonary fibrosis or early-stage interstitial lung disease (ILD). Therefore, clinical correlation and laboratory testing are essential for accurate differentiation.

Clinical Significance and Diagnostic Value of CT Scans

While chest X-rays remain the initial tool for suspected pneumonia, CT imaging offers superior sensitivity and specificity in identifying subtle or atypical presentations. It allows for precise localization, assessment of complications (such as abscess formation, pleural effusion, or cavitation), and monitoring of therapeutic response over time.

Moreover, recognizing characteristic CT patterns helps differentiate between bacterial, viral, and atypical causes of pneumonia, guiding appropriate antimicrobial use and reducing unnecessary treatments. For example, lobar consolidation strongly suggests typical bacterial pathogens like Streptococcus pneumoniae, whereas diffuse interstitial changes point toward viral or mycoplasmal origins.

In summary, CT plays a vital role in modern pneumonia evaluation. By interpreting imaging features in conjunction with clinical symptoms and lab results, healthcare providers can deliver faster, more targeted care—ultimately improving patient outcomes and reducing hospitalization duration.