Group 2: In distinction with the patterns described in patients in group 1, group 2 includes those patients in whom no or very few nodules are perifis-sural or subpleural in distribution. Anatomically, these nodules are grouped together as being centri-lobular in distribution. By definition, these entities primarily involve centrilobular bronchioles and/or their accompanying pulmonary artery branches. Anatomically, these structures taper peripherally, stopping 5 to 10 mm short of the pleural or interlobular septal surfaces and consequently fail to involve pleural and fissural surfaces (Table 1). As will be discussed, these nodules typically fall into the following two broad categories: those with a “tree-in-bud” configuration; and those that appear as amorphous “ground-glass” nodules. Once nodules are characterized as being primarily centrilobular in distribution, further assessment requires determining whether or not these have a tree-in-bud configuration. Tree-in-bud opacities are characterized by the appearance of centrilobular micronodular branching structures that end several millimeters distant from nearby pleural or fissural surfaces (Fig 11). more
Tree-in-bud opacities are nearly always the result of inspissated (ie, frequently aspirated) secretions lodged within centrilobular bronchioles, accounting for a branching configuration when coursing parallel to the CT scan plane. Normal bronchioles, which have a diameter of < 1 mm and a wall thickness of < 0.1 mm, are below the limit of HRCT scan spatial resolution. The presence of inspissated secretions results in bronchiolar distension and increased density, allowing their direct visualization. Not surprisingly, in many cases there is also evidence of coexisting bronchiectasis. Another frequently encountered finding in patients with bronchiolar disease is so-called mosaic attenuation.33 In these cases, bronchiolar occlusion results in air-trapping, hypoxia of the poorly ventilated lung units with resultant reflex vasoconstriction and air-trapping. This combination of findings causes decreased attenuation of the affected areas of the lung with blood flow redistributed to normal lung. The hypoattenuated diseased lung is therefore surrounded by hyperattenuated, overperfused normal lung, resulting in heterogeneous-appearing mosaic attenuation.
Figure 11. Bronchiolar disease: infectious bronchiolitis. A magnified HRCT scan image through the middle and lower lobes shows numerous nodules associated with linear/branching densities throughout the lungs (arrows). These tree-in-bud opacities are the result of infected mucoid impacted peripheral airways and hence have a distinctly centrilobular pattern of distribution. Note that none of these peripherally is in contact with either pleural or fissural surfaces. Classically the result of the endobronchial spread of tuberculosis, this pattern may be seen in virtually any patient in whom there is infection of the peripheral airways. Not surprisingly, tree-in-bud opacities tend to be clustered rather than truly diffuse and frequently are associated with CT scan evidence of bronchiectasis.