Pulmonary edema: hydrostatic and non-cardiogenic
Pathophysiologically, edema is divided into hydrostatic (increased venous pressure, most commonly due to left ventricular failure, or decreased plasma osmotic pressure) and non-cardiogenic (increased permeability of the alveolar-capillary barrier, classic example is ARDS).
Radiographic and CT findings:
- Vascular phase: redistribution of blood flow from the basal regions to the apices. The ratio of artery to bronchus diameters (in cross-section) in the upper lobes increases.
- Interstitial edema: smooth thickening of interlobular septa (on radiographs — Kerley B lines / Kerley B lines), peribronchovascular cuffing. Subtle «ground-glass opacity» appears.
- Alveolar edema (progression): ground-glass areas coalesce, air space consolidation increases.
Additionally, bilateral pleural effusion and reversible enlargement of mediastinal lymph nodes may be detected.
Diffuse pulmonary hemorrhage (DPH)
The imaging appearance in various causes of DPH is stereotyped. Radiographs and CT scans show small centrilobular acinar nodules, uneven consolidation, and areas of «ground-glass opacity». Changes are most frequently localized in the perihilar region of the middle and lower zones.
Key marker: unlike most other opacities (except edema), infiltrates in DPH resolve rapidly within a couple of days as blood is cleared by macrophages. With relapses (e.g., in idiopathic pulmonary hemosiderosis in children or anti-basement membrane antibody disease), fibrosis, septal thickening, and a «crazy-paving» pattern (crazy-paving) develop.
Viral lung involvement
The dynamics of viral pneumonia are stereotyped: initially, peripheral opacities («ground-glass opacity» and consolidation) in the middle and lower zones, which subsequently spread centrally.
Specific CT patterns:
- MERS: bilateral ill-defined areas of consolidation with frequent involvement of the upper lobes.
- Avian influenza (H5N1): multifocal consolidation, frequent formation of pneumatoceles, pseudocavitations, and centrilobular nodules (mortality >60%).
- Swine flu (H1N1): peribronchovascular and subpleural distribution, «reverse halo» sign (ring-shaped consolidation surrounding «ground-glass opacity»), air trapping.
Eosinophilic lung diseases
A group of disorders associated with tissue infiltration by eosinophils. They can be idiopathic, drug-induced (NSAIDs, cocaine, minocycline), infectious (parasites, fungi), or immunologic (Churg-Strauss syndrome).
- Simple eosinophilia (Löffler syndrome): fleeting transient infiltrates disappearing in less than 1 month. Frequently associated with ascariasis.
- Acute eosinophilic pneumonia (AEP): fulminant respiratory distress. According to Philit criteria, duration is ≤1 month. CT findings mimic pulmonary edema or diffuse alveolar damage. Frequently occurs in new smokers.
- Chronic eosinophilic pneumonia: protracted course, focal non-segmental consolidation. A characteristic feature is the «photographic negative of pulmonary edema» sign (opacities located peripherally, parallel to the chest wall).
To avoid confusing the stages of pulmonary edema, remember the path of water from the vessel: first the vascular bed overfills (vascular redistribution), water seeps into the walls (interstitium, Kerley lines), and when the walls fail — it floods the rooms (alveoli, consolidation).
Class and exam questions
Can CT determine the cause of diffuse pulmonary hemorrhage (DPH)?
No, the imaging appearance in DPH is stereotyped. Differential diagnosis is established solely based on clinical presentation, laboratory data (e.g., antibodies in Goodpasture syndrome), and medical history.
How to differentiate pulmonary hemorrhage from viral pneumonia on serial X-rays?
Changes in intra-alveolar hemorrhage resolve very rapidly (within a few days) due to the action of pulmonary macrophages. Inflammatory viral infiltrates persist for weeks.
Which CT sign is considered pathognomonic for organizing pneumonia?
Perilobular distribution of consolidation areas. Organizing pneumonia is also characterized by linear opacities associated with bronchi (Type I) or running parallel to the pleura (Type II).
What else to study on this topic
- Philit criteria for the diagnosis of acute eosinophilic pneumonia.
- Changes in the imaging pattern of infections in HIV patients undergoing antiretroviral therapy (ART).
- Immune reconstitution inflammatory syndrome (IRIS) in mycobacterial infections.
- Idiopathic pulmonary hemosiderosis: pathogenesis and prognosis.
- Classification of linear opacities in organizing pneumonia patterns.
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