Chest ImagingAir trapping
a pathophysiological term indicating the retention of excess gas in all or part of the lungs, at any stage of expiration. On the chest radiograph,
diffuse air trapping is recognized as a decreased excursion of the
diaphragm during the respiratory cycle. At expiration the
diaphragm is in a lower position than normally expected. When air trapping is only
focal, localized to one lung or one lobe, the
radiographic findings visualized on the chest film performed at suspended expiration are those of
overinflation. The lung areas of air trapping appear relatively increased in volume by contrast to the normally ventilated areas, and hyperlucent with
pulmonary vessels reduced in number and in size.
On inspiratory chest radiographs, the areas of air trapping may appear normal or slightly decreased in size. On CT scan, a lack of increase in density at expiration in any region, together with a lack of decrease in cross-section of that region, indicates air trapping. Areas of air trapping can be patchy and nonanatomical, can correspond to individual secondary pulmonary lobules, segments or lobes, or may involve an entire lung. Pulmonary vessels within low-attenuation areas of air trapping often appear small relative to vessels in the more attenuated normal lung. When expiratory air trapping is diffuse, the high resolution computed tomography (HRCT) scans performed at expiration may be unremarkable. The decreased lung attenuation is uniform, and the most striking features are the paucity of pulmonary vessels, and the lack of change in lung cross-section surfaces between inspiration and expiration.
Air trapping is a finding characteristic of obstruction of the airways. It may be present in all types of bronchiolar disease and bronchial obstruction of any cause. It may also be seen in emphysema, chronic obstructive pulmonary disease and asthma, and in some infiltrative lung diseases such as hypersensitivity pneumonitis and sarcoidosis. Abnormal air trapping on expiratory HRCT scans seen in the absence of inspiratory abnormalities is most often associated with constrictive bronchiolitis and asthma, and is also frequently observed in healthy smokers. Areas of air trapping are reported to show an increased attenuation of less than 100 Hounsfield units (HU) on dynamic expiratory scans, a value that is significantly lower than that of normal lung. Measurement of the change in overall lung attenuation from inspiration to expiration may be used in patients with diffuse air trapping, but is clearly less sensitive than in patients with patchy disease.
The extent of air trapping on CT scans may be assessed by a visual score or pixel index (Fig.1). The quantitative assessment of lung attenuation changes during or following expiration can be measured using a variety of methods including spirometrically triggered CT and dynamic CT performed using electron beam or helical scanner.
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