Radiological presentation of severe emphysema
On standard chest radiographs, pulmonary hyperinflation manifests as downward displacement and flattening of the diaphragmatic domes. In the frontal view, the diaphragmatic contours may appear irregular due to the visualization of their attachments to the ribs. The heart acquires a specific shape — the «teardrop heart».
At the periphery of the lungs, attenuation (depletion) of the vascular pattern is noted. In the lateral view, expansion of the costophrenic angle and an increase in the size of the retrosternal transradiant area (retrosternal transradiant area) are characteristic.
CT subtypes of emphysema
Computed tomography allows reliable classification of emphysema at the lobar level of the lung:
- Centrilobular emphysema: multiple rounded areas of decreased attenuation, localized predominantly in the upper lobes around centrilobular arteries. As the disease progresses, these zones coalesce, and at the stage of advanced destructive emphysema, hyperinflation of the secondary lobules and architectural distortion occur. Vessels appear attenuated and straightened (pruned-tree sign).
- Panlobular emphysema: associated with genetic $\alpha$1-antitrypsin deficiency. The lower lobes are predominantly affected, showing a generalized decrease in tissue attenuation. Specific CT marker: the volumetric enlargement of the lower lobes causes the left main interlobar fissure to bulge anteriorly.
- Paraseptal emphysema: well-demarcated subpleural lucencies along the pleura and interlobar fissures. If the size of a cyst-like lucency exceeds 1 cm, it is termed a bulla.
- Bullous emphysema: multiple large avascular areas. If bullae occupy more than 1/3 of the hemithorax, it is referred to as a giant form, which can cause compressive atelectasis of the adjacent parenchyma.
Small and large airway involvement
In COPD patients, CT reveals bronchial wall thickening (relative to the diameter of the adjacent artery) and signs of small bronchiole involvement.
At the early (inflammatory) stage, areas of ground-glass opacity and small centrilobular nodules are visible in the upper lobes. The late stage (obstructive bronchiolitis) is characterized by mosaic attenuation and expiratory «air trapping» (air trapping). Normally, lung tissue attenuation increases on expiration, but this does not occur in areas of terminal bronchiolar obstruction.
Patients also frequently present with bronchial diverticula — herniation-like outpouchings of bronchial glands through the smooth muscle (best visualized in minIP mode). Expiratory central airway collapse (tracheal lumen narrowing by 50% on expiration) and tracheobronchomalacia (narrowing by more than 80%) may occur.
Associated findings
In addition to parenchymal destruction, COPD is frequently associated with additional abnormalities:
- Interstitial abnormalities: gravity-independent areas of ground-glass opacity and reticular changes. They can visually mimic the honeycombing pattern characteristic of interstitial pneumonias.
- Pulmonary hypertension: dilatation of the main pulmonary artery trunk. If its diameter exceeds that of the aorta, it is considered an independent predictor of future hospitalizations.
- Bronchiectasis: frequently of the tubular (cylindrical) type in the lower lobes, which correlates with more severe obstruction.
Briefly: answers to topical questions
What complications are characteristic of giant bullae in bullous emphysema?
Giant bullae in bullous emphysema can lead to compression of surrounding structures and the development of several pathological conditions. The following complications are characteristic:
- Organ compression — compression of the lung (with the development of compressive or relaxation atelectasis), diaphragm, and right atrium (with the risk of tamponade).
- Pneumothorax — accumulation of air in the pleural cavity.
- Hemorrhage — from the cavity of the bulla.
- Infection — development of an inflammatory process within the bulla.
What quantitative CT criteria and attenuation thresholds are used to objectively assess the severity of emphysema?
To objectively assess the severity of emphysema, quantitative computed tomography (QCT) lung densitometry (lung densitometry) is used, based on the segmentation of voxels with an X-ray attenuation below a specified threshold. The following criteria are used:
- -950 HU threshold — the most commonly used metric in clinical practice, providing an optimal balance of sensitivity and specificity.
- -960 and -970 HU thresholds — demonstrate the highest correlation with histological data on multislice CT.
- Percentile density (percentile density) — the attenuation value below which a specified percentage of pixels lies. The standard metric for detecting disease progression is the 15th percentile.
Emphysema localization rule: CENTER (centrilobular) affects the UPPER lobes (typical for smokers), while PAN (panlobular) destroys the LOWER lobes ($\alpha$1-antitrypsin genetic deficiency).
Class and exam questions
How to reliably detect small airway obstruction on CT?
To do this, an expiratory CT scan must be performed (at full expiration). Obstruction manifests as air trapping — areas where tissue attenuation does not increase on expiration and the physiological anteroposterior attenuation gradient is absent.
What is the pruned-tree sign?
This is a CT sign of advanced destructive emphysema, in which pulmonary vessels on maximum intensity projection (MIP) images appear attenuated, straightened, and their peripheral branching is markedly depleted.
What is the clinical significance of expiratory central airway collapse?
A reduction in the cross-sectional area of the trachea on expiration by 50% or more is associated with a poorer patient quality of life and a higher frequency of acute respiratory events.
What else to study on this topic
- Quantitative computed tomography (QCT) image analysis and selection of reconstruction algorithms for emphysema assessment
- Expiratory CT technique and calculation of expiratory air trapping severity
- Differential diagnosis of paraseptal emphysema and cystic bronchiectasis
- Visualization of bronchial diverticula using minimum intensity projection (minIP)
- CT criteria for pulmonary hypertension in COPD and measurement of the PA/AA ratio
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