Optimal CT scanning techniques
To reliably detect air trapping, specialized protocols must be used. The primary tool is full-expiration computed tomography (expiratory CT). On expiratory slices, healthy lung parenchyma decreases in attenuation, whereas affected areas retain gas. This dramatically increases contrast and makes hypoventilated zones clearly visible.
If a patient has difficulty holding their breath at end-expiration, a dynamic expiratory maneuver is performed during spiral scanning. This increases the degree of expiration and improves the detection of air trapping.
The routine approach in multidetector CT (MDCT) involves thin collimation and low or ultra-low radiation doses. Images are processed using iterative reconstruction or filtered back-projection with a soft tissue kernel. A critical role belongs to the minimum intensity projection (Minimal Intensity Projection, minIP). Combined with volumetric multiplanar reconstruction, it brilliantly demonstrates the mosaic perfusion pattern (mosaic perfusion) typical of patients with bronchiolitis obliterans.
Quantitative and semi-quantitative assessment
In clinical practice and research, the severity of air trapping must be quantified. Two main approaches are used for this purpose:
- Semi-quantitative Stern scoring system (Stern et al.). The percentage of affected lung is evaluated per slice using a 4-point scale:
- 0 points — no air trapping;
- 1 point — 1 to 25% of tissue;
- 2 points — 26 to 50%;
- 3 points — 51 to 75%;
- 4 points — 76 to 100%.
Points are summed up. This system demonstrates good inter- and intra-observer agreement and correlates excellently with the degree of obstruction measured by pulmonary function tests (PFT).
- Objective measurement (CT densitometry). This is an automated calculation using the density mask technique (density mask technique). The software segments pixels in air-trapping zones based on a threshold of –910 HU. This method allows calculation of expiratory-inspiratory indices, comparison of full inspiration and expiration, and generation of a clear 3D lesion model.
Association with COPD and bronchiolar pathology
Air trapping is one of the fundamental CT signs of chronic obstructive pulmonary disease (COPD). COPD arises as an excessive inflammatory response to tobacco smoke and pollutants, combining parenchymal destruction (emphysema) and small airway narrowing.
Bronchiolar pathology begins with inflammation in and around the walls (respiratory bronchiolitis). Subsequently, the lumen becomes plugged with exudate and mucus plugs. In advanced stages, peribronchiolar fibrosis develops, leading to irreversible narrowing — bronchiolitis obliterans.
In patients with bronchiectasis, the presence of expiratory air trapping, alongside decreased lung attenuation and bronchial wall thickening, is the primary determinant of obstruction. Notably, wall thickening is considered the main factor driving subsequent lung function decline.
Role of emphysema in air trapping development
Emphysema (emphysema) is the permanent enlargement of airspaces distal to the terminal bronchioles, accompanied by wall destruction without overt fibrosis. It is important to note that the narrowing and loss of terminal bronchioles precede microscopic emphysematous destruction.
According to the protease-antiprotease theory, an imbalance of enzymes (e.g., due to $\alpha_1$-antitrypsin deficiency or smoking) leads to the destruction of elastic fibers. This results in decreased elastic recoil and reduced radial traction of the bronchial walls. Consequently, airways collapse, exacerbating air retention (air trapping).
CT allows phenotyping of COPD and differentiation of emphysema patterns:
- Centrilobular: ill-defined localized areas of decreased attenuation without visible walls, typically in the upper lobes.
- Paraseptal: affects alveoli along the septa and subpleurally, often forming bullae.
- Panlobular: enlargement of all spaces within the secondary pulmonary lobule, a severe type with basal predominance.
Briefly: answers to topical questions
Which diseases should be differentiated from the mosaic perfusion pattern on CT?
The mosaic perfusion pattern must be differentiated between airway diseases and chronic occlusive vascular diseases.
The main conditions for differential diagnosis include:
- Bronchiolitis obliterans (obliterative bronchiolitis) — characterized by heterogeneous bronchiolar involvement and air trapping.
- Chronic thromboembolic pulmonary hypertension (CTEPH) — characterized by hypoperfusion in the vascular beds of obstructed vessels.
- Panlobular emphysema (panlobular emphysema) — presents a CT appearance similar to bronchiolitis obliterans.
- Hypersensitivity pneumonitis (hypersensitivity pneumonitis) — manifests as lobular areas of decreased vascularity.
What direct CT signs of small airway disease are visualized on inspiratory scans?
On inspiratory scans, small airway disease manifests as decreased lung attenuation and specific nodular patterns.
The following direct CT signs are distinguished:
- Areas of decreased X-ray attenuation (decreased lung attenuation) — accompanied by a reduction in pulmonary vessel caliber.
- Centrilobular nodules (centrilobular nodules) — may have a branching appearance, forming the tree-in-bud sign (tree-in-bud).
- Paucity of the vascular pattern (paucity of pulmonary vessels) — vessels are reduced in number and caliber.
Wall thickening and bronchial dilatation may also be visualized.
Which specific enzymes cause the destruction of elastic fibers according to the protease-antiprotease theory of emphysema?
The destruction of elastic fibers is caused by proteolytic enzymes whose activity increases uncontrollably due to inhibitor deficiency.
Destructive enzymes include:
- Neutrophil elastase (serine proteinase) — a broad-spectrum serine protease acting as the main driver of destruction.
- Cathepsins (cathepsins) — belong to the group of cysteine proteases.
- Matrix metalloproteinases (matrix metalloproteinases) — enzymes that degrade extracellular matrix components.
These enzymes are actively secreted by phagocytes (macrophages and neutrophils) in response to chronic inflammation.
How do air trapping areas differ from ground-glass opacity zones?
Air trapping and ground-glass opacity reflect opposite changes in the X-ray attenuation of lung tissue.
| Characteristic | Air trapping (air trapping) | Ground-glass opacity (ground-glass opacity) |
|---|---|---|
| X-ray attenuation | Decreased (hypoventilated zones, pixels < –910 HU) | Increased (opacification of airspaces) |
| Pathophysiological basis | Increased aeration due to hyperinflation | Alveolar septal thickening, edema, fibrosis |
| Imaging conditions | Better visualized on expiratory CT (exhalation) | Visualized on standard scans (acute phase) |
For the Stern scoring system (evaluating trap volume), calculate in quarters: 1 point — a quarter (25%), 2 points — half (50%), 3 points — three quarters (75%), 4 points — the entire lung (100%).
Class and exam questions
Why should CT for detecting air trapping be performed on exhalation?
Upon exhalation, healthy tissue densifies and loses aeration, while areas of air trapping remain distended. This creates contrast (mosaic perfusion) and allows hypoventilation to be visualized.
What density threshold is used for automated assessment?
To isolate air-trapping pixels using the density mask method, a threshold value of –910 HU is applied.
What is minimum intensity projection (minIP)?
This is a CT reconstruction algorithm that highlights pixels with the lowest density, making areas of air trapping and the mosaic perfusion pattern more prominent.
Which small airway pathology most frequently presents with a trapping pattern?
Bronchiolitis obliterans — narrowing of the terminal bronchiole lumen due to peribronchiolar fibrosis, leading to irreversible air trapping.
What else to study on this topic
- COPD pathogenesis: protease-antiprotease theory
- Classification of centrilobular emphysema (Fleischner Society)
- Radiological signs of hyperinflation (lung hyperinflation)
- Giant bullous emphysema and vanishing lung syndrome
- Assessment of bronchiectasis and their impact on lung function
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