Expiratory High-Resolution CT in Diffuse Lung Diseases

High-resolution computed tomography (CT) of the lung is a powerful diagnostic tool in revealing morphological abnormalities that closely correspond to those of the pathological tissue specimen. High-resolution CT is usually obtained at the end of deep inspiration; however, additional CT obtained at the end of deep exhalation (expiratory CT) provides a different set of information. The lung usually shows a homogeneous increase in lung density after forced exhalation, and the cross-sectional area decreases accordingly in healthy subjects; however, those areas peripheral to diseased airways often show no increase in lung density or decrease in cross-sectional area after exhalation. These areas (airtrapping areas) are seen in various diffuse lung diseases, including small airway diseases, some interstitial lung diseases, and even in normal subjects. Irrespective of the disease or condition, the extent of air-trapping approximately correlates with obstructive functional impairment. In patients with suspected lung parenchymal disease, air-trapping can be the only imaging abnormality; thus, expiratory CT can visualize early or mild parenchymal lung disease before the development of overt abnormalities. In this manuscript, the various techniques of paired inspiratory and expiratory high-resolution CT are reviewed and the normal appearances of expiratory images are described. Correlation of imaging findings with pulmonary function tests is discussed and the clinical impact of the technique is reviewed for some of the diffuse parenchymal lung diseases.