Anatomical background
Accurate diagnostic imaging requires an understanding of the structural organization of the lungs. Air spaces (air spaces) refer to the air-containing part of the organ, which includes respiratory bronchioles and alveoli. Terminal bronchioles are excluded, as they serve merely as final pathways for air conduction.
Basic structural units:
- Acinus (acinus) — a portion of lung tissue ventilated by exactly one terminal bronchiole.
- Secondary pulmonary lobule — the smallest unit surrounded by connective tissue septa. At its center lie the lobular bronchiole and accompanying artery.
Normally, on high-resolution computed tomography (HRCT), only centrilobular arteries up to 0.2 mm in diameter can be seen. The wall of the bronchiole itself is too thin for visualization. If it becomes visible (especially subpleurally), this is a reliable sign of pathology—thickening, dilation, or mucus accumulation.
Principles of diagnostic imaging
Chest radiography always remains the first and basic imaging method. Computed tomography is ordered to clarify the diagnosis, detect cavitations, or search for specific signs (e.g., the "crazy-paving" pattern in alveolar proteinosis).
To narrow down the differential diagnosis, the radiologist evaluates three key factors:
- Clinical presentation. The same bilateral opacity may indicate acute respiratory distress syndrome in a critically ill patient or an extensive infection in a patient with fever and cough.
- Distribution of changes. In cryptogenic organizing pneumonia, infiltrates tend to be peripheral and lower-zone predominant, whereas in chronic eosinophilic pneumonia, they are more frequently located in the upper zones, parallel to the chest wall.
- Temporal dynamics. Rapid resolution of infiltrates over a few hours or days points toward edema or pulmonary hemorrhage. If opacities are transient and "migratory," eosinophilic pneumonia should be suspected.
Main radiological patterns
The main challenge in diagnostic imaging is that many different diseases produce the same picture. Three leading patterns of air space involvement are distinguished:
- Nodular pattern. Rarely occurs in isolation. Previously, the terms "acinic rosettes" or "acinic nodules" were used to describe poorly defined foci in bacterial infections and hemorrhages.
- Ground-glass opacity (ground-glass opacification). This is a hazy increase in tissue density. On a standard X-ray, it completely obscures vessel contours, while on CT, the vascular and bronchial markings remain discernible.
- Consolidation (consolidation). Dense opacification of lung tissue against which vessels and airways are completely invisible. Often accompanied by the air bronchogram sign.
Causes of opacities
The replacement of air in the lungs can occur due to numerous causes, which are classified into several groups:
| Pathology group | Examples of diseases |
|---|---|
| Pulmonary edema | Cardiogenic and non-cardiogenic (hydrostatic or due to increased vascular permeability) |
| Inflammation | Diffuse alveolar damage, vasculitis, organizing pneumonia |
| Hemorrhages | Systemic lupus erythematosus, idiopathic hemosiderosis |
| Neoplasms | Lymphoma, adenocarcinoma with lepidic growth |
Rarer causes also include alveolar microlithiasis, sarcoidosis, and eosinophilic lung diseases.
Briefly: answers to topical questions
Which specific neoplasms can manifest with an air space disease pattern?
The following neoplasms can manifest with an air space disease pattern:
- Adenocarcinoma with lepidic growth pattern (Adenocarcinoma with lepidic growth pattern), formerly known as bronchioloalveolar carcinoma.
- Lymphoma (Lymphoma), specifically MALT lymphoma.
Which diseases are characterized by a peripheral subpleural distribution of infiltrates?
A peripheral subpleural distribution of infiltrates is characteristic of:
- Cryptogenic organizing pneumonia (Cryptogenic organising pneumonia) — areas of consolidation are more frequently located in the periphery, predominantly in the middle and lower zones; subpleural curvilinear bands may also be present.
- Chronic eosinophilic pneumonia (Chronic eosinophilic pneumonia) — infiltrates in the upper zones, located peripherally and parallel to the chest wall, are typical.
What does the "crazy-paving" sign on CT represent morphologically?
Morphologically, the "crazy-paving" sign (crazy-paving pattern) on computed tomography represents a combination of ground-glass opacities with pathological thickening of interlobular and intralobular septa. In its classic form, this radiological pattern is considered pathognomonic for pulmonary alveolar proteinosis. In addition, it can be observed in patients with diffuse pulmonary hemorrhage, alveolar microlithiasis, and late stages of viral lung injury.
To narrow down the differential diagnosis for opacities, use the three-pillar rule: "CDD" — Clinic, Distribution, Dynamics.
Class and exam questions
What is the "air bronchogram" sign and what is its purpose?
This is a diagnostic CT sign in diffuse ground-glass disease. It occurs due to a sharp contrast between the air within the bronchial lumen (appearing black) and the denser, infiltrated surrounding lung tissue.
How does cardiogenic pulmonary edema differ from non-cardiogenic edema?
Cardiogenic edema occurs due to elevated hydrostatic pressure in the pulmonary circulation. Non-cardiogenic edema is associated with increased permeability of the pulmonary capillaries themselves.
What does the air bronchogram sign mean?
This is a sign of consolidation in which dark strips of bronchi that are still filled with air become visible against the airless, dense lung tissue.
What else to study on this topic
- Pulmonary alveolar proteinosis and the "crazy-paving" pattern
- Acute respiratory distress syndrome (ARDS): imaging findings
- Cryptogenic organizing pneumonia and perilobular distribution
- Chronic eosinophilic pneumonia and the phenomenon of migratory infiltrates
- Adenocarcinoma with lepidic growth pattern: diagnostic challenges
Master the entire topic in the "Diagnostic Radiology" course
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Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.