Pathophysiology and main CT signs
In bronchial asthma, regional differences in small airway closure lead to a specific radiological pattern. On computed tomography, this manifests as mosaic perfusion (mosaic perfusion) and air trapping (gas trapping).
For a long time, it was believed that air trapping was caused exclusively by smooth muscle spasm. However, studies have shown that in patients with moderate persistent asthma, mosaic perfusion and air trapping may persist without significant changes even after full bronchodilator inhalation. This fact proves the leading role of structural airway remodeling (airway remodelling).
In mild persistent disease, the gold standard for imaging is axial CT performed on a breath-hold strictly at end-expiration. Such scans clearly visualize patchy (focal) areas of air trapping. As a rule, the pathological process involves individual pulmonary lobules and segments, with changes localized predominantly in the lower lobes of both lungs, as well as in the middle lobe of the right lung.
Quantitative computed tomography (QCT)
The introduction of multidetector row CT (multidetector row CT) has made precise assessment of obstruction severity possible. The method of pulmonary densitometry allows for quantitative analysis of expiratory scans by calculating the percentage of pixels whose X-ray attenuation falls below a specified threshold—so-called low attenuation areas.
For standardized assessment of air trapping in clinical practice, the following indices are used:
- Threshold value of -850 HU when scanning at functional residual capacity (FRC) level.
- Threshold value of -900 HU when scanning at full expiration.
- The ratio of mean lung parenchyma density at expiration to the same parameter at inspiration.
- The absolute difference between lung tissue density at inspiration and expiration.
- The lowest 10th percentile of the frequency distribution of lung density at expiration.
Quantitative assessment is of immense clinical significance. The severity of air trapping significantly correlates with airway hyperresponsiveness, disease duration, and airflow limitation. Patients with a pronounced CT pattern have a significantly higher risk of hospitalizations, intensive care unit (ICU) admission, and the need for mechanical ventilation. In addition, densitometry is successfully used to monitor response to inhaled corticosteroid therapy.
Imaging of severe asthma and 3D modeling
According to quantitative CT, the morphology of the proximal airways in severe asthma is characterized by two fundamental features: airway wall thickening (airway wall thickening) and airway lumen narrowing (airway lumen narrowing).
Wall thickening correlates with asthma control level and the severity of air trapping. In severe disease, bronchial walls are significantly thicker than in healthy individuals. CT indices such as percentage wall thickness (%WT) and percentage wall area (%WA) closely correlate with pathomorphological biopsy findings.
The area of the bronchial lumen in such patients is significantly narrowed. To detail morphological remodeling, 2D and 3D analysis of wall volume, luminal narrowing, and bronchial stenoses is used. Evaluation of dynamic changes (lumen delta) is performed by comparing scans at full inspiration and full expiration.
Modern software allows for automatic segmentation of the inner and outer contours of the trachea and bronchi with the construction of their 3D models. An example is the sagittal 3D representation of the right bronchial tree. Using color coding of wall thickness (on a scale from < 0.01 mm to > 1.5 mm), one can clearly see the colossal difference between a healthy person and a patient with severe asthma, who exhibits pronounced wall thickening and lumen narrowing. The use of such CT indices in combination with air trapping assessment allows for the identification of asthma phenotypes with distinct clinical characteristics.
Briefly: answers to topical questions
What structural airway changes are detected on CT in severe bronchial asthma?
In severe bronchial asthma, computed tomography reveals signs of proximal airway remodeling. The main structural changes include:
- Airway wall thickening (airway wall thickening) — this sign correlates with bronchial hyperresponsiveness, airflow limitation, and the presence of air trapping.
- Airway lumen narrowing (airway lumen narrowing) — the bronchial lumen area in such patients is significantly narrowed compared to healthy individuals.
- Bronchial stenoses (bronchial stenoses) — evaluated during 2D and 3D analysis of morphological remodeling of large airways.
Which conditions must be ruled out when air trapping is detected on expiratory CT?
When air trapping is detected on expiratory CT, the pathology must be differentiated from other obstructive and interstitial diseases. Conditions accompanied by this symptom include:
- Chronic obstructive pulmonary disease (COPD) is characterized by expiratory air trapping and various subtypes of emphysema.
- Constrictive (obliterative) bronchiolitis (obliterative bronchiolitis) — manifests as extensive areas of decreased density on expiration; can be post-infectious, toxic, or idiopathic.
- Panlobular emphysema (panlobular emphysema) — requires differential diagnosis with obliterative bronchiolitis due to a similar CT pattern.
- Hypersensitivity pneumonitis (hypersensitivity pneumonitis) — air trapping reflects concomitant bronchiolitis.
To interpret CT in severe asthma, remember the "Remodeling Triad": thick wall, narrow lumen, expiratory air trapping.
Class and exam questions
At which respiratory phase should CT be performed to detect air trapping?
Scanning must be performed during a breath-hold at end-expiration (expiratory CT). It is precisely in this phase that unevenly closed small bronchi form visible areas of air retention (air trapping).
Why do air trapping symptoms in asthma not always disappear after bronchodilator administration?
The persistence of mosaic perfusion and air trapping after bronchodilator inhalation indicates that they are based not only on reversible smooth muscle spasm, but also on structural airway remodeling.
How does the CT pattern affect the disease prognosis?
The presence of pronounced air trapping is associated with a more severe course of asthma. In such patients, the risk of hospitalizations, ICU admission, and the need for mechanical ventilation increases significantly.
What else to study on this topic
- Calculation of %WT (percentage wall thickness) and %WA (percentage wall area) indices
- Algorithms for 2D and 3D analysis of bronchial stenoses and wall volume
- Identification of severe asthma phenotypes (clusters) based on the CT pattern
- Methodology for assessing bronchial lumen delta at full inspiration and expiration
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