Lobar pneumonia is characterized histopathologically by filling of alveolar airspaces by an exudate of edema fluid and neutrophils. This filling is usually uniform within the affected lung and typically extends across pulmonary segments. The consolidation usually begins in the periphery of the lung adjacent to the visceral pleura and spreads via inter-alveolar pores and small airways centripetally, sometimes to involve the entire lobe. Bronchi that remain filled with gas and become surrounded by the expanding inflammatory exudate are often seen as air bronchograms on CT scans. The most common causative organisms are Streptococcus pneumoniae, Klebsiella pneumoniae (Fig. 26.1), and Legionella pneumophila.

On CT scans, homogeneous airspace consolidation involving adjacent segments of a lobe is the predominant finding. On HRCT, areas of ground-glass opacities (GGO) denoting incomplete filling of alveoli can be seen adjacent to the airspace consolidation [1]. The consolidation typically extends across lobular and segmental boundaries.

Segmental or lobar form of tuberculous pneumonia may occur in adults; in AIDS or non-AIDS immunocompromised patients including pregnant women, elderly, diabetics, alcoholics, and transplanted patients. Multiple small cavities may be shown within the consolidative lesion [2, 3] (Figs. 26.2 and 26.3).

Bronchopneumonia is characterized pathologically by patchy, predominantly peribronchiolar inflammation. The reason why this localization is different from lobar pneumonia is unclear but may be related to relatively less abundant edema formation (associated with more difficult spread of infection within the lung) and more virulent organisms (resulting in greater tissue destruction) in bronchopneumonia. Although initially patchy, progression of disease may result in lobular and segmental consolidation (Fig. 26.4). The main causative organisms are Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa, and anaerobic bacteria.

Characteristic findings on HRCT include centrilobular small nodules and branching linear structures, airspace nodules, and multifocal lobular consolidation or GGO [1] (Fig. 26.5). The small nodules and branching linear opacities result in an appearance resembling a tree-in-bud and are related to the presence of the inflammatory exudate in the lumen and walls of membranous and respiratory bronchioles and the lung parenchyma immediately adjacent to them (Fig. 26.6). This appearance is similar to cellular bronchiolitis caused by viruses, Mycoplasma pneumoniae and Chlamydia [1, 4]. Unlike the latter (atypical pneumonias), however, the tree-in-bud pattern in bacterial bronchopneumonia usually comprises only a small proportion of the HRCT abnormalities. In addition, thickening of bronchovascular bundles is more frequently observed in atypical pneumonia, in which individual opacity tends to be distributed at the inner zone in addition to the middle and outer zones and be recognized smaller than those of bacterial pneumonias [1] (Fig. 26.5).