Pneumonia is an infection that causes inflammation, primarily affecting the small air sacs (alveoli) in the lungs. When the alveoli fill with fluid, pus, and inflammatory cells, it impairs oxygen exchange, leading to symptoms. While a standard chest X-ray is often the first diagnostic step, a computed tomography (CT) scan is a more advanced imaging tool. CT scans are utilized when the diagnosis is uncertain or when complications are suspected, providing highly detailed cross-sectional images to visualize the precise location and extent of the infection.
The Role of CT Imaging in Pneumonia Diagnosis
CT imaging provides a detailed view of lung tissue often obscured on a chest X-ray. Studies show CT scans can reveal infiltrates in many patients, sometimes up to 38%, who had a normal or inconclusive chest X-ray. This superior sensitivity is useful in populations like the elderly or those with underlying pulmonary disease, where subtle signs of infection might be missed on less detailed images.
The cross-sectional nature of the CT scan allows physicians to map the infection in three dimensions, determining the spread and severity of the disease. It helps differentiate pneumonia from conditions that mimic its appearance, such as atelectasis (lung collapse) or a pulmonary tumor. When a patient is not responding to initial antibiotic treatment, a CT scan is invaluable for identifying secondary complications, which often require a change in management strategy.
Fundamental Visual Signatures of Pneumonia
The most common visual marker of pneumonia on a CT scan is consolidation, which appears as a dense, white, uniform area of opacity in the lung tissue. This whiteness indicates that the air normally filling the alveoli has been replaced by inflammatory fluid, pus, or blood. The dense infiltrate obscures the normal pulmonary blood vessels, eliminating the contrast that makes them visible.
A second common finding is ground-glass opacity (GGO), which presents as a hazy, faint increase in lung density. Unlike consolidation, GGO does not completely obscure the underlying bronchial structures or blood vessels, allowing them to remain visible. GGO often indicates inflammation or partial filling of the air spaces, frequently seen in early-stage, viral, or atypical infections.
A third, highly specific signature often seen within consolidation is the air bronchogram sign. This appears as dark, air-filled tubes (the bronchi) running through the surrounding bright, white consolidated tissue. Air bronchograms confirm that the large, proximal airways remain open and air-filled, despite the surrounding alveoli being filled with fluid. This finding suggests the density is due to airspace filling (like pneumonia) rather than a mass or complete bronchial obstruction.
Differentiating Pneumonia Types by CT Pattern
The distribution of consolidation and ground-glass opacity helps narrow down the potential cause of the infection. Lobar pneumonia is characterized by homogeneous consolidation that respects the anatomical boundaries of a lung lobe or segment. This pattern suggests the infection, often caused by bacteria like Streptococcus pneumoniae, spread rapidly through the pores connecting the air sacs.
In contrast, bronchopneumonia (lobular pneumonia) appears as patchy, scattered areas of consolidation centered around the airways, often involving multiple lobes and appearing bilaterally. This pattern arises when the infection spreads via the bronchial tree. The resulting CT image shows a non-contiguous, multi-focal distribution of opacities, common in infections caused by organisms like Staphylococcus aureus or Klebsiella.
A third pattern, interstitial pneumonia, primarily involves the supporting tissue of the lung (the interstitium) rather than the air sacs. This is often caused by viruses or atypical bacteria and is predominantly seen as ground-glass opacity or a fine, net-like pattern called reticular thickening. Recognizing these distinct patterns helps guide the initial selection of treatments, as the pattern of disease spread correlates with the likely causative pathogen.
Recognizing Severe Complications on CT Scans
CT scans are highly effective at identifying severe complications. One common complication is pleural effusion, the accumulation of fluid in the pleural space surrounding the lung. On a CT scan, this appears as a crescent-shaped collection of fluid density outside the lung parenchyma, which may need drainage if large or infected.
A more serious development is a lung abscess, a localized pocket of pus and necrotic debris. This appears on CT as a thick-walled, round or oval cavity, often containing both air and fluid, creating a distinct air-fluid level. The thick wall helps distinguish an abscess from other types of lung cavities.
Necrotizing pneumonia represents tissue death within the consolidated lung area, a severe form of the infection. On CT, this manifests as multiple small areas of low attenuation (darker areas) within the bright consolidation. These areas of non-enhancement indicate that the tissue is no longer receiving adequate blood flow or has been destroyed by the infection.