Pneumonia

Lobar Pneumonia

Lobar pneumonia is characterized histopathologically by Þlling of alveolar airspaces by an exudate of edema ßuid and neutrophils. This Þlling 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 Þlled with gas and become surrounded by the expanding inßammatory 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 Þnding. On HRCT, areas of ground-glass opacities (GGO) denoting incomplete Þlling 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).

b

Fig. 26.1 Klebsiella pneumonia in a 62-year-old woman. (a) Mediastinal window of enhanced CT scan (2.5-mm section thickness) obtained at level of liver dome shows dense lobar consolidation in both lower lobes containing multifocal areas of necrotic low-attenuation areas (arrows). Also note CT air-bronchogram signs (arrowhead) within consolidative lesions. (b) Coronal reformatted image demonstrates necrotic lobar consolidation involving both lower lobes. Also note right pleural effusion (open arrows)

a

b

Fig. 26.2 Tuberculous pneumonia manifesting as segmental consolidation in a 26-year-old woman. (a) Mediastinal window of enhanced CT scans (5.0-mm section thickness) obtained at level of basal trunk shows segmental consolidation (arrows) in right middle lobe. Also note enlarged right hilar lymph node (open arrow). (b) CT scan obtained at level of distal left main bronchus demonstrates enlarged and necrotic subcarinal lymph node (arrows) and right hilar node (open arrow)

Bronchopneumonia

Bronchopneumonia is characterized pathologically by patchy, predominantly peribronchiolar inßammation. 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 difÞcult 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,

a

b

Fig. 26.3 Lobar consolidation as a manifestation of pulmonary tuberculosis in a 24-year-old man. (a) Mediastinal window of enhanced CT scans (5.0-mm section thickness) obtained at level of main bronchi shows lobar consolidation (arrows) in left upper lobe. Some parenchymal lesions are also seen in right lung. Note enlarged subcarinal lymph node (open arrow) and left pleural effusion (arrowheads). (b) Lung window image obtained at the same level to a demonstrates adds Þndings of tree-in-bud signs and small nodules in contralateral right lung suggestive of bronchogenic spread of tuberculous pneumonia

Haemophilus influenzae, Pseudomonas aeruginosa, and anaerobic bacteria.

Characteristic Þndings 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 inßammatory 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

Fig. 26.4 Pulmonary blastomycosis manifesting as morphologic bronchopneumonia in a 45-year-old man. (a) Lung window image of thin-section (1.5-mm section thickness) scan obtained at level of main bronchi shows combination of subsegmental consolidation (arrows), acinus-sized small nodules (arrowheads), smaller nodules, and ground-glass opacity in right upper lobe. (b) CT scan obtained at level of basal trunk demonstrates nodular clustering (arrowheads) in right

middle lobe. (c) Gross pathologic specimen obtained with right upper lobectomy depicts mainly consolidative lesion, but also small nodular lesions (arrowheads). Lesion has multifocal areas of small abscess (arrows). (d) Low-magniÞcation (×4) photomicrograph discloses suppurative granulomatous inßammation. Inset: double-walled yeast in cytoplasm of multinucleated giant cell consistent with blastomycosis

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).

Pulmonary tuberculosis also presents with prototypical morphologic dense bronchopneumonia. On CT, the lesion consists of tree-in-bud pattern, acinar nodule, lobular consolidation, and cavitating or noncavitating nodules or subsegmental or segmental consolidation. Bronchial wall thickening may be associated [2] (Fig. 26.6).

Fig. 26.5 Mycoplasma pneumoniae pneumonia in a 29-year-old man who received hematopoietic stem cell transplantation owing to acute myeloid leukemia. (aÐc) CT scans (2.5-mm section thickness) obtained at levels of aortic arch (a), bronchus intermedius (b), and basal trunks (c), respectively, show lung abnormalities comprising tree-in-bud signs (arrows), small nodules (arrowheads) and ground-glass opacity lesions (open arrows) in both lungs. Please note involvement of both central and peripheral portions of lungs

Fig. 26.6 Bronchopneumonia in tuberculous infection in a 26-year- old man. (a) Lung window image of CT scan (5.0-mm section thickness) obtained at level of right upper lobar bronchus shows extensive bronchopneumonia consisting of cavitary consolidation (arrow), variable-sized nodules (arrowheads), and dilated bronchi in right upper lobe. Also note bronchogenic spread of infection to contralateral left lung (open arrows). (b) CT scan obtained at level of liver dome demonstrate less extensive bronchopneumonia involving right lower and middle lobes. Abnormality contains lobular consolidation (arrows), variable-sized nodules (arrowheads), and tree-in-bud signs (open arrows). (c) Gross pathologic specimen obtained with right pneumonectomy discloses tuberculous abscess (Ab) of yellow creamy necrotic materials seen as consolidation on CT scan, consolidation, variablesized nodules, and nodular branching structures (arrowheads) shown as tree-in-bud signs at CT