CAP has classically been divided into three distinctive patterns on imaging examinations, namely consolidation (alveolar/lobar pneumonia), peribronchial nodules (bronchopneumonia) and ground-glass opacity (GGO)[4,10]. The fourth, a unique uncommon pattern of CAP is random nodules, suggestive of hematogenous pulmonary infection or granulomatous infection.

In fact, many pathogens can cause pneumonia with more than one pattern. In addition, consolidation, peribronchial nodules and GGO can often coexist in a case of pneumonia although one of these findings usually predominates. Virulence, amount or size of pathogens, affinity to certain cells, and immune response of hosts may relate to the different manifestations of CAP on imaging examinations. However, the reason why CAP has different patterns of imaging findings is unknown.

Consolidation predominant pneumonia is referred to as alveolar pneumonia (Figures 1-4). When it affects almost an entire lung lobe, it is called “lobar pneumonia”. This consolidation is believed to be formed by the spread of inflammation through pores of Kohn or canals of Lambert at the periphery of the lung. Thus, it usually appears in a nonsegmental consolidation in the early stage of disease[10]. Most bacterial pneumonias exemplified by Streptococcus and Klebsiella pneumonia appear in consolidation predominant pattern[4].

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Figure 1 Streptococcus pneumoniae pneumonia showing alveolar pneumonia in a man in his 80s. A: Chest radiograph shows a nonsegmental consolidation in the right middle lung field, which is demarcated by the minor fissure suggestive of upper lobe pneumonia (arrow); B, C: Thin-section computed tomography (B) and a coronal reformatted image (C) demonstrate a nonsegmental consolidation with air bronchograms suggestive of alveolar pneumonia (arrows).

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Figure 2 Mycoplasma pneumoniae pneumonia showing alveolar pneumonia in a woman in her 30s. A: Chest radiograph demonstrates ill-defined consolidation in the right lower lung field (arrow); B: Thin-section CT reveals a non-segmental consolidation with air bronchograms at the dorsal aspect of the right lower lobe. Areas of ground-glass opacity are also noted around the consolidation (arrows). CT: Computed tomography.

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Figure 3 Chlamydophila pneumoniae pneumonia showing alveolar pneumonia in a man in his 60s. A: Chest radiograph shows an ill-defined consolidation at the right lower lung field (arrow); B: On thin-section CT, a subpleural focal consolidation is seen at the right S8 of the right lower lobe, partially extending into the middle lobe (black arrow). The interlobular fissure is mildly thickened (arrow heads). Mild bronchial wall thickening is also noted (white arrow). CT: Computed tomography.

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Figure 4 Tuberculous pneumonia in a woman in her 80s. A: Chest radiograph shows extensive consolidations with poor aeration of the left lung and peribronchovascular consolidations of the right lung (arrows); B: Thin-section computed tomography reveals extensive consolidation with air bronchograms and cavities in the left upper lobe (black arrows). Note that the bronchi in the consolidation are dilated. Dense centrilobular nodules are seen in the left lower lobe (white arrows).

This pattern is characterized by the predominance of peribronchial nodules including centrilobular nodules with or without peribronchial consolidations (Figure 5)[4,10]. In contrast to consolidation predominant pattern, these consolidations are probably formed by enlargement and coalescence of the peribronchial nodules. Bronchial wall thickening is often associated. Pneumonia with this pattern is called bronchopneumonia. However, bronchopneumonia is sometimes indistinguishable from alveolar pneumonia. When centrilobular nodules predominate, namely when bronchioles and peribronchiolar areas are mainly affected, it may be referred to as infectious bronchiolitis (Figures 6-9)[4]. Hemophilus influenzae, Mycoplasma pneumoniae, Chlamydophila pneumoniae, and viruses are the representative pathogens of this disease entity[4]. Tuberculosis and atypical mycobacterial infection also fall in this category. However, in fact, most pathogens can take this pattern of pneumonia[4].

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Figure 5 Mycoplasma pneumoniae pneumonia showing bronchopneumonia in a man in his 10s. A: Chest radiograph shows reticulonodular opacities and focal consolidation in the left middle to lower lung field (arrow). The left pulmonary hilum appears enlarged; B: Thin-section computed tomography demonstrates fluffy centrilobular nodules with surrounding ground-glass opacity in the left lower lobe (arrows). Note that central bronchial wall is thickened (arrow heads).

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Figure 6 Postprimary tuberculosis in a woman in her 40s. A: Chest radiograph shows faint nodular opacities in the right middle lung field (arrow). There is also volume loss of the left lung with patchy consolidations and thickening of the pleura and possible left pleural effusion, indicative of old tuberculosis; B: Thin-section computed tomography demonstrates centrilobular branching opacities (tree-in-bud appearance) in the right upper lobe (arrows). The branching opacities are denser, more distributed and more peripherally located than those of ordinary bronchopneumonia (compare with Figure 3).

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Figure 7 Chlamydophila pneumoniae pneumonia showing infectious bronchiolitis in a woman in her 60s. A: Chest radiograph shows faint reticulonodular opacities in both lower lung fields (arrows); B: Thin-section computed tomography reveals centrilobular nodules (arrows) with bronchiectasis (arrow heads) in the middle lobe and lingula.

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Figure 8 Chronic transbronchial infection (diffuse aspiration bronchiolitis) in a man in his 70s. This patient had a history of esophageal carcinoma and associated repeated aspiration. Thin-section computed tomography at the level of lung base shows centrilobular nodules (arrows) with bronchiectasis (arrow heads). Low attenuation areas suggestive of pulmonary emphysema are also present (*).

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Figure 9 Sinobronchial syndrome in a man in his 70s. A: Chest radiograph shows bilateral reticulonodular opacities in both lower lung fields (arrows); B: computed tomography (CT) at the level of maxillary sinus demonstrates opacification of the maxillary sinuses (*) and bone sclerosis of the sinus walls (arrows), suggestive of chronic paranasal sinusitis; C: Thin-section CT reveals bronchial wall thickening with bronchiectasis (arrows) and minimal centrilobular opacities (arrow heads).

Bronchopneumonia may follow a chronic clinical course. In such a case, bronchiectasis, prominent reticular opacities or architectural distortion, indicative of chronic process of the disease, are usually present (Figures 7-9).

Streptococcus pneumoniae pneumonia is the most common CAP, accounting for 40% of CAP[4]. It usually appears in alveolar/lobar pneumonia on chest radiograph and CT (Figure 1)[4,14-16]. Lower lobe is preferentially involved but multi-lobe involvement is also common[14]. Bilateral lung disease is seen in about half of cases[14].

Mycoplasma pneumoniae pneumonia commonly affects young people[4,16]. It is clinically characterized by dry cough, fever, and general fatigue[8]. Chest radiograph shows reticulonodular opacities or patchy consolidations[17]. On CT, centrilobular nodules and lobular to acinar areas of consolidation or GGO with bronchial wall thickening are the most common findings (Figure 5)[4,14,16,17]. These findings are consistent with bronchopneumonia. The bronchial wall thickening is often seen in central bronchi. This finding may be related to the fact that Mycoplasma pneumoniae targets bronchial epithelium[14]. Bronchopneumonia with central bronchial wall thickening in children and young adults are fairly specific findings for Mycoplasma pneumonia. However, extensive GGO (Figure 10) or consolidation (Figure 2) is also not uncommon[16]. GGO predominance in Mycoplasma pneumoniae pneumonia may represent permeability edema rather than cellular infiltrates with edema. Acute respiratory distress syndrome may ensue[18].

It has been well known that Chlamydophila pneumoniae pneumonia often appears as part of co-infection. Therefore, strict diagnostic criteria should be used to diagnose this pneumonia. We have been using the diagnostic criteria for acute infection of Chlamydophila pneumoniae using ELISA kit established by Kishimoto et al[19,20]; Chlamydophila pneumoniae pneumonia is considered to be present when IgA or IgG index exceeds over 3.0, or there is interval increase more than 1.0 in IgA or 1.35 in IgG in paired serum specimens. These criteria were proved to be accurate for acute Chlamydophila pneumoniae infection, yielding a specificity of 93.4% (i.e., 7.6% of healthy population shows more than this cut-off value) and sensitivity of 64.9%[19].

Chest radiograph shows patchy consolidations or reticular opacities (Figure 3, Figure 7)[21]. At the time of reinfection, reticular opacities predominate[21]. On CT, various patters are seen, including alveolar pneumonia (Figure 3), bronchopneumonia (Figure 7) and GGO predominant pneumonia (Figure 11)[16,22]. Consequently, imaging findings of Chlamydophila pneumoniae pneumonia is virtually non-specific. However, bronchopneumonia or infectious bronchiolitis in elderly patients with pulmonary emphysema or other chronic debilitating lung disease may be one of the characteristic manifestations of Chlamydophila pneumoniae pneumonia[16].

Legionella pneumophila pneumonia is a fatal pneumonia, and therefore, early diagnosis and treatment is crucial[23]. Chest radiographic findings include unilateral nonsegmental poorly defined airspace consolidation[24]. CT findings consist mainly of consolidation and GGO[25]. Bilateral lung disease is seen in two thirds of cases[25]. It has been reported that sharply marinated peribronchial consolidations within GGO are characteristic of Legionella pneumonia seen in about one third of the cases (Figure 12)[25].

There are innumerable causative viruses for pneumonia. Therefore, the imaging findings of virus pneumonia are diverse. Viral pneumonia can virtually take any form of the above mentioned patterns. Among them, bronchopneumonia and GGO predominance are the most common presentations of viral pneumonia (Figure 13)[4]. Random nodule pattern is characteristic of varicella-zoster (VZ) pneumonia (Figure 15)[11] although military tuberculosis or mycosis, or bacterial emboli shares this finding. It is conceivable that random nodules seen in VZ pneumonia are due to the fact that VZ hematogenously infects the lung[26]. Scattered small nodules with calcification are also sometimes seen in patients with a past history of VZ pneumonia[11,26].

Mixed infection, namely accompanying bacterial pneumonia, is also common in viral pneumonia. In such a case, consolidation often predominates. However, pure viral pneumonia may also demonstrate consolidations. Therefore, it is difficult to make a diagnosis of mixed infection with imaging examinations alone.

Aspiration pneumonia is caused by inhalation of bacteria, food, gastric acid or other materials that provoke pulmonary inflammation or edema (e.g., paraffin liquid)[37], Therefore, aspiration pneumonia has several different pathophysiological aspects, namely bacterial pneumonia caused by oral flora (usually anaerobic bacteria), chemical pneumonitis caused by gastric acid or exogenous lipid, and granulomatous reaction to foreign bodies[37]. Aspiration pneumonia commonly occurs in patients with deteriorated consciousness, chronic debilitating disease, and tracheal or gastric tubes[37-40]. Therefore, it more commonly appears in hospital-acquired pneumonia[37-40]. However, postoperative status for esophageal or gastric cancer and reflux esophagitis are also known risk factors of aspiration pneumonia and thus aspiration pneumonia may manifest as CAP[37-40].

It commonly affects dorsal parts of the lung (S2, S1+2, S6 and S10) and demonstrates findings of bronchopneumonia or infectious bronchiolitis (Figure 19)[37-40]. Aspirated materials are sometimes seen in the bronchial lumens (Figure 19)[40]. Patchy GGO with peribronchial distribution are also common manifestation (Figure 20). This finding is considered to be related to permeability edema due to endothelial injury by aspirated gastric acid. Acute respiratory distress syndrome (ARDS) may result from aspiration and is referred to as Mendelson’s syndrome[41].

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Figure 19 Aspiration pneumonia in a woman in her 70s (causative pathogen unknown). Thin-section computed tomography images show centrilobular nodules with surrounding ground-glass opacities and subpleural non-segmental consolidations (arrows) at the dorsal portions of the right lung. Note that the lumens of segmental bronchi are filled with aspirated materials (arrow heads).

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Figure 20 Pneumonia caused by aspiration of gastric fluid in a man in his 50s. A: Initial thin-section computed tomography (CT) shows patchy ground-glass opacity in the left lung with reticulations; B: Thin-section CT 2 d later demonstrates partly increased attenuation with concaved margin of the opacities as well as a general resolution of pneumonia.

Chronic infectious bronchiolitis radiologically mimicking diffuse panbronchiolitis is named diffuse aspiration bronchiolitis (Figure 8)[42].

Sinobronchial syndrome is defined as chronic and repeated infection of the lower respiratory tract and paranasal sinuses, which includes diffuse panbronchiolitis[43]. It was once believed to be caused by aspiration of purulent discharge in the paranasal sinuses. However, altered immune status is now considered to lead to both paranasal sinusitis and infectious bronchiolitis[43]. Chest radiograph shows reticulonodular opacities with lower lung field predominance (Figure 9). On CT, centrilobular or peribronchial nodules with bronchial wall thickening and mucus in the bronchi are seen (Figure 9). There are usually evidences of chronic and repeated infection, such as bronchiectasis, parenchymal distortion or reticular opacities. Comparison with previous imaging examinations is essential to make a diagnosis of acute exacerbation of infection.

When pneumonia develops on a background of pulmonary emphysema, parenchymal consolidations caused by pneumonia appear to have multiple cavities due to underlying low attenuation areas (Figure 21)[7,10]. This appearance is referred to as “Swiss cheese appearance”[10]. If low attenuation areas predominate, it may mimics honeycombing (Figure 22). These pseudocavitations and pseudohoneycombing must be distinguished from true cavities and honeycombing. Also, resolution is delayed in pneumonia associated with pulmonary emphysema. It should also be noted that infection is the most common cause of acute exacerbation of chronic obstructive pulmonary disease[44].

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Figure 21 Pneumonia on a background of pulmonary emphysema in a man in his 70s (causative pathogen unknown). Thin-section computed tomography shows patchy consolidations with small air-containing spaces consistent with preexistent low attenuation areas in the left lower lobe.

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Figure 22 Pneumonia on a background of pulmonary emphysema mimicking honeycombing in a man in his 60s (causative pathogen unknown). Thin-section computed tomography shows an extensive area of ground-glass opacity intermingled with consolidation in the right lower lobe. Pseudohoneycombing is seen along the interlobar fissure (arrows).

There is no imaging finding that is 100% specific for infectious pneumonia. Consolidation and GGO are virtually non-specific. However, peribronchial nodules, especially tree-in-bud appearance are fairly specific for infection[4]. When viewed with CT, consolidation, GGO and peribronchial nodules are coexistent in most cases of infectious pneumonia. Therefore, peribronchial nodules can often be a diagnostic clue for infectious pneumonia.

Cryptogenic organizing pneumonia (COP) (formerly bronchiolitis obliterans organizing pneumonia) is clinically characterized by dry cough and dyspnea that continue for a couple of months[45]. A typical clinical scenario is that the respiratory symptoms do not improve despite medication with antibiotics[45]. On imaging examinations, it typically shows patchy consolidations which sometimes predispose peribronchial areas with or without extensive areas of GGO[44-47]. Nodules are not uncommon[45-48]. There are often evidences of organization, such as concavity of the opacities, traction bronchiectasis, visualization of air bronchograms over the entire length of the bronchi, or mild parenchymal distortion (Figure 23). Reversed halo sign or atoll sign is also suggestive of COP (Figure 24)[48]. This sign indicates a central GGO surrounded by a ring of consolidation with a thickness of 2 mm or more (just like the reverse of halo sign or atoll in the sea)[48]. It is seen in 20% of patients with COP[48]. Although this sign was first considered specific for COP, it has been shown that other diseases may demonstrate reversed halo sign since then. These diseases include invasive pulmonary fungal infections, paracoccidioidomycosis, Pneumocystis jirovecii pneumonia, tuberculosis, lymphomatoid granulomatosis, Wegener granulomatosis, lipoid pneumonia and sarcoidosis. It is also seen in pulmonary neoplasms and infarction, and following radiation therapy and radiofrequency ablation of pulmonary malignancies[49,50]. However, as it has not been reported that reversed halo was seen in CAP so far, it is considered useful to differentiate COP from CAP[50].

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Figure 23 Cryptogenic organizing pneumonia in a woman in her 50s. A: Chest radiograph shows a consolidation in the right lower lung field with depression of the right minor fissure suggestive of volume loss of the middle lobe (white arrow). Retrocardiac consolidation is marginally seen (black arrow); B, C: Thin-section computed tomography of the right lung (B) and left lung (C) demonstrate consolidations with air bronchograms in both lungs. Note that the bronchi within the consolidations are mildly dilated and that the consolidations have concaved margins (arrows), suggesting organization of the disease.

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Figure 24 Cryptogenic organizing pneumonia with reversed halo sign in a woman in her 50s: courtesy of Dr. Takahiro Haruyama and Dr. Asako Yamamoto, attending radiologists at the department of Radiology, Teikyo University School of Medicine. A: Chest radiograph shows an ill-defined consolidation in the left lower lung field (arrow); B: Thin-section computed tomography reveals bibasilar ground-glass opacity with a ring of consolidation (reversed halo sign (arrows).

Organizing pneumonia that is histologically identical to COP may develop secondary to infectious pneumonia, organ transplantation, drug use or in association with collagen vascular disease[45]. The imaging findings are the essentially the same as COP. However, associated findings related to the primary diseases may be seen, such as findings of bronchopneumonia or honeycombing.

Chronic eosinophilic pneumonia (CEP) is defined as eosinophilic pneumonia for more than 2 wk and is the most common subtype in eosinophilic lung diseases[51,52]. It is clinically characterized by eosinophilia, and coexistence of atopic medial otitis and asthma[51,52].

Chest radiograph typically shows bilateral nonsegmental consolidations with peripheral predominance[51,52]. This appearance is referred to as “the photographic negative of pulmonary edema” (Figure 25)[51,52]. On CT, bilateral or unilateral peripheral consolidations and GGO are seen[47,53]. Linear or band-like opacities parallel to the pleura may be seen at the later stage of the disease[47,53]. CEP may mimic COP. Thickening of interlobular septa is more commonly seen in CEP, whereas nodules and peribronchial distribution of the opacities are more common in COP[47].

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Figure 25 Chronic eosinophilic pneumonia in a woman in her 30s. Chest radiograph shows bilateral subpleural consolidations with upper to middle lung field predominance, consistent with the appearance, “the photographic negative of pulmonary edema”.

Invasive mucinous adenocarcinoma (formerly mucinous bronchioloalveolar carcinoma) and malignant lymphoma may appear in alveolar consolidations, and thus may mimic alveolar pneumonia (Figures 26, 27)[54-56]. These neoplasms lack evidence of inflammation on the laboratory data, or if any, the values of inflammatory markers are milder than expected from the extent of disease on imaging examinations. On CT, bronchi in the consolidation are stretched or narrowed, and the consolidation may have a bulging contour at the interlobar fissures in invasive mucinous adenocarcinoma (Figure 26)[55]. It has also been reported that focal areas of the parenchymal opacification on CT may suggest infectious pneumonia rather than invasive mucinous adenocarcinoma when they show bronchial wall thickening proximal to the lesion and pleural thickening associated with the lesion, whereas invasive mucinous adenocarcinoma is characterized as the presence of a bubble-like low attenuation area within the tumor[56].

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Figure 26 Invasive mucinous adenocarcinoma (formerly mucinous bronchioloalveolar carcinoma) in a woman in her 60s. Thin-section computed tomography shows a nonsegmental consolidation with a bulging fissure (white arrow) and narrowed air bronchograms (black arrows).

Malignant lymphoma often assumes an infiltrative growth, which may appear as halo sign (ground-glass opacities around the nodule or consolidation)[57] or as surrounding miliary nodules or thickening of surrounding vessels (Figure 27).

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Figure 27 Pulmonary malignant lymphoma (mucosa associated lymphoid tissue lymphoma) in a woman in her 30s. Thin-section computed tomography shows a focal consolidation with dilated air bronchograms in the lingula of the left upper lobe. Note mild thickening of the vessels penetrating the consolidation (arrows), suggestive of infiltrative growth of malignant lymphoma along the vessels.

Lipoid pneumonia is divided into endogenous and exogenous type. Exogenous lipoid pneumonia results from the chronic aspiration or inhalation of animal, vegetable or petroleum-based oils or fats[58]. Exogenous lipoid pneumonia is considered as a subtype of aspiration pneumonia. CT shows consolidations and GGO with reticular opacities (crazy-paving appearance). The consolidations may have CT values indicative of fat (-150-300HU) (Figure 28)[57]. Visual assessment for the presence of fat is also essential as consolidation even without fat may apparently have areas of low CT value comparable to that of fat due to volume averaging between air and inflammatory infiltrates or exudate.

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Figure 28 Exogenous lipoid pneumonia in a woman in her 30s: Courtesy of Dr. Kazuhiro Suzuki, an attending radiologist, at the department of Radiology, Juntendo University School of Medicine. This patient had been taking petrolatum (paraffin) for intractable constipation. The presence of lipid was confirmed by transbronchial lung biopsy. A: Chest radiograph shows bilateral consolidations in the lower lung fields (arrows); B: Thin-section CT demonstrates an area of clearly demarcated ground-glass opacity with a subpleural consolidation (arrow); C: Chest computed tomography (CT) with a mediastinal window setting reveals the subpleural consolidation to be of fat attenuation (arrow, mean CT value -45HU).