Pneumonia

Fundamentals of diagnostic imaging

The primary indication for chest imaging is the presence of physical signs of lung tissue involvement. The goal of imaging is to confirm or rule out focal pneumonia and identify complications.

A plain chest radiograph typically shows bronchial wall thickening and enhanced pulmonary markings, but computed tomography (CT) has significantly higher specificity. CT is mandatory for patients with a high clinical and laboratory probability of the disease if infiltrates are not visible on the radiograph or the picture is ambiguous.

Viral pneumonias form a specific set of features on CT:

• Bilateral involvement.
• Diffuse or patchy ground-glass opacity.
• Consolidation of the lung parenchyma.
• Reticular changes and interlobular septal thickening («crazy-paving» pattern).
• «Reverse halo» sign (or atoll sign).

Pneumonias in systemic autoimmune diseases

Autoimmune pathologies are frequently accompanied by pulmonary involvement, the radiological presentation of which can mimic an infection.

In polymyositis and dermatomyositis, weakness of the respiratory muscles comes to the forefront. This provokes aspiration pneumonia, a complication with high morbidity and mortality. Additionally, 47% of patients develop interstitial lung disease (ILD). On high-resolution CT (HRCT), lower lobe involvement predominates with areas of ground-glass opacity (corresponding to nonspecific interstitial pneumonia — NSIP) and consolidation (organizing pneumonia).

In systemic lupus erythematosus (SLE), pulmonary changes are extremely variable. Most commonly, a small bilateral pleural effusion is detected radiologically. Significant diagnostic challenges arise from the detection of consolidation zones. These can be a consequence of superimposed infection in the setting of immunosuppression, lupus pneumonitis, pulmonary edema, or diffuse alveolar hemorrhage. Acute lupus pneumonitis is rare and presents with fever, hypoxemia, and imaging findings of focal consolidation and basal focal atelectasis.

Specific pneumonias: tuberculosis

Mycobacterial infections are overwhelmingly (>95%) caused by M. tuberculosis.

Primary tuberculosis is frequently detected in adults today (23–34% of cases). On a radiograph, up to 15% of patients may show no pathology at all. In other cases, homogeneous lobar consolidation—visually indistinguishable from bacterial pneumonia—and lymphadenopathy are detected (especially in children). Following resolution, one-third of patients retain a calcified Ghon focus (Ghon lesion). Together with the calcified regional lymph node, it forms the Ranke complex (Ranke complex).

Secondary tuberculosis (reactivation) proceeds with a more pronounced inflammatory response. Lymphadenopathy is not characteristic, whereas upper lobe involvement and active cavity formation are typical (in 40–80% of cases). Endobronchial spread of the infection produces the characteristic «tree-in-bud» pattern (tree-in-bud) on CT. One of the rare life-threatening complications is Rasmussen's aneurysm (granulomatous weakening of the pulmonary artery wall near a cavity, detected on contrast-enhanced scans).

Immune reconstitution inflammatory syndrome (IRIS) deserves special mention. This is a paradoxical worsening of the radiological picture (appearance of pulmonary nodules, areas of organizing pneumonia, enlargement of lymph nodes with central hypodensity) in response to antigen as CD4 counts rise. It must not be mistakenly interpreted as an infection relapse.

Class and exam questions

What else to study on this topic

• Radiological diagnosis of diffuse alveolar damage (DAD)
• Radiological patterns of organizing pneumonia
• Differential diagnosis of lupus pneumonitis
• Rasmussen's aneurysm: CT diagnosis of a life-threatening complication
• Shrinking lung syndrome in systemic lupus erythematosus

Other topics in the «Chest Radiology» section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.