Air trapping can be seen on expiratory images as an accentuation of density between adjacent areas of the lungs. When attenuation is measured, areas of air trapping maintain their low attenuation values, whereas normal lung increases in density. This is typically a manifestation of small airway disease but can result from either luminal obstruction or concentric bronchial wall thickening [3].

Architectural distortion refers to the displacement of normal lung structures with disruption of the usual lung parenchyma. It is generally associated with a fibrosing lung process (Fig. 2.2).

Atelectasis is the loss of air and volume caused by the collapse of the airspaces (Fig. 2.3). It is therefore the opposite of airspace disease.

Bronchiectasis is defined as irreversible dilation of the airways usually measured as 1.5× the size of the adjacent artery [4]. It can be cylindrical (long and tubular), varicoid (variable lumen size), or cystic (large gas-filled spaces) (Fig. 2.4). The relatively small artery next to a dilated bronchus has been referred to as the “signet ring sign.”

Traction bronchiectasis is a unique subset of bronchiectasis and is related to underlying parenchymal fibrosis [5]. As opposed to bronchiectasis described above, the dilation is the result of external retractile forces. When severe, it can be quite difficult to segregate out from honeycomb lung.

Centrilobular nodules are hazy, ill-defined nodules that are centered around the central bronchus and artery in the secondary pulmonary lobule (Fig. 2.5). These nodules do not extend out to the interlobular septum and are classically associated with subacute hypersensitivity pneumonitis and respiratory bronchiolitis [1].

Crazy-paving pattern combines ground-glass opacity superimposed upon interlobular septal thickening somewhat resembling paved cobblestones. Although classically associated with alveolar proteinosis, it can occur as part of any disease process that involves both interstitium and air spaces [6].

Ground-glass opacity is used to describe a hazy increase in lung attenuation that does not obscure the underlying lung architecture (see Fig. 2.5) [7]. It can be associated with airspace-filling process or fibrosis that is below the resolution of CT. Because of this, the moniker of “active alveolitis” is discouraged, although this may represent reversible cellular change in the setting of interstitial lung disease.

Honeycombing is a late radiographic finding of underlying fibrosis with stacked thick-walled cystic structures (see Fig. 2.2). In the classic basal and peripheral distribution, it helps to define a usual interstitial pneumonitis pattern and obviates the need for open lung biopsy [8]. It is important to recognize that honeycombing present within a pathologic specimen may not always be evident at TSCT.

Interlobular septal thickening refers to any process that results in increased conspicuity of the walls of the secondary pulmonary lobule [1]. This may alternatively be described as reticular or reticulonodular opacities depending on morphology. Best seen perpendicular to the pleural surface, smooth interlobular septal thickening (Fig. 2.6) or reticular opacities are a classic manifestation of hydrostatic pulmonary edema. A beaded or reticulonodular appearance can be seen with such conditions as sarcoidosis and lymphangitic carcinomatosis.

Mosaic attenuation is a general term used to describe differing lung densities. As such it can be due to a mixture of ground-glass opacity and normal areas of air trapping and normal lung or areas of differential (mosaic) perfusion [9]. The former two possibilities can be sorted out with expiratory images as both ground-glass disease and normal lung will both increase in density, whereas air trapping will manifest as persistent low density. Mosaic perfusion can be differentiated by looking at the underlying vessels with larger vessels going to areas of increased density and small or diminutive vessels going to areas of decreased density as in cases of chronic thromboembolic pulmonary hypertension.

Peribronchovascular nodule describes small nodules (1–3 mm) that are discretely related to the bronchial wall or peribronchial structures (Fig. 2.7). It therefore may be associated with disease of the bronchus, adjacent vessels, or lymphatics. Nodules of this variety may also be described as bronchocentric.