Bronchiectasis
An enhanced and distorted pulmonary pattern serves as the radiological trigger for a targeted search for bronchiectasis. A characteristic distinguishing feature is that pathological changes are localized in the lower lung fields and can be traced all the way to the peripheral zones. Often, radiographs additionally reveal signs of atelectasis in the lower and middle lobes, as well as in the lingular segments.
The patient's corresponding clinical presentation should prompt a targeted examination. In the early stage, when bronchiectasis has not yet formed morphologically, the disease mimics chronic bronchitis with intermittent cough and mucopurulent sputum. The medical history of such patients often includes recurrent lower respiratory tract infections, episodes of hemoptysis, and a doubtful diagnosis of "asthma" requiring careful differential diagnosis.
Interstitial pneumonia
In interstitial lung diseases, the radiological picture is fundamentally different. Against the background of signs of emphysema (hyperinflation of the lung tissue), an enhanced and distorted reticular interstitial pattern is clearly visualized. In some cases, large focal shadows may form, creating the characteristic "snowflake" sign.
The disease is typical for infants in the first months of life, while in older patients it proceeds as an opportunistic infection against the background of immunodeficiency. The causative agents are most often viruses, mycoplasmas, chlamydia, and Pneumocystis. The condition is characterized by the rapid development of respiratory failure, cyanosis, exhaustive unproductive cough, as well as a high risk of pneumofibrosis and acute cor pulmonale.
Hemodynamic disorders of the pulmonary circulation
Changes in the pulmonary vascular pattern often accompany mitral stenosis. With an increase in left atrial pressure, venous congestion occurs. In the initial stages, radiographs show significant dilation of the pulmonary veins. Blood flow is redistributed to areas with initially lower circulation (the lung apices), while perivascular edema develops in areas with rich blood supply.
If the pulmonary capillary wedge pressure exceeds 18–25 mmHg, pulmonary arterial hypertension joins the congestion. This is the result of reflex spasm of small arteries and arterioles (Kitaev's reflex). At this stage, radiography reveals intensive homogeneous opacification and enlargement of the lung roots with a loss of border definition. The pulmonary trunk enlarges and pulsates intensely, whereas at the periphery the vascular pattern sharply diminishes due to the constriction of small arteries.
CT capabilities in diagnosing bronchiolar involvement
When small airways are involved (e.g., in constrictive obliterating bronchiolitis), plain chest radiography is insufficient. Thin-section computed tomography detects the pathology much more accurately. Scans obtained during inspiration reveal areas of decreased lung tissue attenuation with a significant reduction in vessel caliber.
Heterogeneous airway obstruction leads to blood flow redistribution to healthy lung zones, forming a mosaic perfusion pattern. Key distinguishing feature: in areas of decreased attenuation, vessel caliber is sharply reduced, but the vessels are not distorted. To confirm the diagnosis, expiratory CT is performed, where heterogeneity is enhanced due to the visualization of "air trapping" — affected areas remain lucent, while healthy tissue predictably densifies.
For differential diagnosis: enhancement and distortion in the lower zones extending to the periphery point to bronchiectasis; a diffuse distorted net indicates interstitial involvement; redistribution to the apices and blurred roots are consequences of venous congestion.
Class and exam questions
What does the "snowflake" sign mean on a chest radiograph?
This is the appearance of large focal shadows against the background of an enhanced reticular interstitial pattern and lung hyperinflation. This sign is characteristic of interstitial pneumonia.
How does the pulmonary pattern change in Kitaev's reflex?
Due to reflex arteriolar spasm, the vascular pattern at the periphery of the lungs becomes sparse. At the same time, the lung roots, pulmonary veins, and pulmonary trunk are significantly dilated.
Why is expiratory CT needed in obliterating bronchiolitis?
During expiration, healthy tissue becomes denser, while affected areas do not change attenuation due to air trapping. This sharply increases contrast and reliably reveals mosaic perfusion.
What else to study on this topic
- Clinical and radiological differential diagnosis of atypical and destructive pneumonias.
- Causes and forms of obliterating bronchiolitis (including Swyer-James syndrome).
- Pathophysiology of mosaic attenuation and mosaic perfusion on CT.
- Complications of destructive pneumonias and outcomes of lung abscess.
- Specific pathogens of pneumonia with rapid development of respiratory failure.
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