Focal opacity may be visible, especially when comparing one lung with the other on the frontal projection. On the lateral projection attention should be directed over the thoracic spine, the cardiac silhouette, and the retrosternal and retrocardiac regions, where faint opacity may otherwise escape detection (Figs. 5.1 and 5.2).

Lung opacity contacting a mediastinal or diaphragmatic border will obliterate or silhouette that border (“silhouette sign”). Right middle lobe opacity can be subtle on the frontal CXR, necessitating careful scrutiny of the right heart border (Fig. 5.3). Lingular disease obliterates the left heart border (Fig. 5.4), whereas lower lobe opacity may obscure a hemidiaphragm.

This describes replacement of the airspaces with fluid or exudate without gross destruction or displacement of lung morphology. Signs of airspace disease (Chapter 14) (Fig. 5.5)
occur. At CT early airspace disease may manifest as ground glass opacity, with increased attenuation but with pulmonary vessels still visible.

Perceptible thickening of a bronchus is most commonly seen in viral infections. It can often be discerned by comparing the bronchus diameter with its accompanying pulmonary artery.

This describes loss of lung volume (Fig. 5.6), and the term collapse is used when a whole lobe or lung is involved. Mild atelectasis occurs in many patients, especially those with interstitial pneumonia. It usually manifests as linear or discoid atelectasis.

This gram-positive coccus is responsible for anywhere from 10% to 80% of cases of CAP (9,10). It is the most frequent organism resulting in hospitalization for pneumonia. It is common in healthy young adults, typically presenting with chills, fever, and cough productive of rust-colored blood-tinged sputum. Spread is by the airborne route. Diagnosis can be made by sputum or blood culture, although pneumococci are found in the sputum of 10% to 40% of normal patients. Sputum cultures are negative in nearly half of patients with positive blood cultures. Most patients are diagnosed presumptively, based on clinical and radiographic presentation, without the need for cultures.

Homogeneous airspace disease confined to a single lobe with an irregular margin (Fig. 5.16) is the most common radiographic pattern, seen in about one-third of patients. Pneumococcal pneumonia is one of the causes of expansive consolidation with bowing of fissures. More widespread patchy bronchopneumonia is seen in another one-third, usually confined to a single lobe. Interstitial opacification simulating viral or atypical pneumonia occurs in one-fourth of patients. The remainder show a mixed patchy and interstitial pattern. There is a predilection for the lower lobes. Pleural effusions are seen in one-third. Consolidation in pneumococcal pneumonia is said to clear by central “lysis.” Patients with shorter clinical histories tend to show more rapid radiographic resolution. As in all pneumonias there is a lag period between clinical resolution and radiologic resolution, which can be several weeks.

This gram-positive coccus is an uncommon but serious cause of CAP (11,12,13). It has a fulminant course with fever, cough, dyspnea, purulent sputum, hemoptysis, and chest pain. Spread occurs by the airborne or hematogenous route. When acquired via inhalation, S. aureus pneumonia is often a complication of influenza during epidemics. Airborne spread
is also implicated in debilitated patients. Hematogenous spread occurs secondary to soft tissue infections, valve prostheses, hemodialysis, and intravenous drug use. Morbidity is high, with a reported mortality rate of 7% to 19%.

Initially, radiographs most commonly (75%) show multilobar homogeneous airspace disease (Fig. 5.17). Subsequent radiographic deterioration is often seen. Bilateral changes are noted in 35%. Cavitation or abscess formation (25%), pneumatoceles (40%), pleural
effusions (33%), and pneumothorax (20%, often associated with pleural effusion or empyema) are other common findings. Pneumonia with pneumatocele or pneumothorax should suggest S. aureus as the cause.

This is a gram-positive sporulating nonmotile organism. It is endemic in the soil of Texas, Oklahoma, and the lower Mississippi valley (14). Cutaneous disease accounts for over 90% of cases, with gastrointestinal and inhalational disease causing 5% each. Inhalational disease results from industrial or agricultural exposure to animal hides, hair, wool, or bone meal of contaminated livestock. The disease is most prevalent among herbivores such as cattle, sheep, horses, and goats. Prior radiation exposure, alcoholism, and underlying pulmonary disease are thought to be risk factors. Spores of 2 to 5 μm reach the alveoli. Macrophages engulf the spores and transfer them to mediastinal and hilar nodes. Germination occurs with production of large amounts of anthrax toxin. This spills over into the systemic circulation with resultant edema, hemorrhage, necrosis, and septic shock. Massive hemorrhagic mediastinitis occurs. Inhalational anthrax manifests as an initial flu-like illness followed by rapidly progressive respiratory failure and death.

CXRs show widening of the mediastinum due to hemorrhagic mediastinitis and lymphadenitis (15,16). Pleural effusions also occur. Focal hemorrhagic pneumonia is only seen in one-fourth of patients. CT better demonstrates hemorrhagic mediastinitis and necrosis. It also excludes other etiologies of widened mediastinum.

This gram-negative bacillus is also known as Friedlander bacillus (17). It is found in the upper respiratory tract of 2% to 25% of healthy persons. It is an uncommon cause of pneumonia and a well-known cause of biliary and urinary tract sepsis. Middle-aged men
are most commonly affected, and many patients have predisposing conditions such as alcoholism, malnutrition, and diabetes. Chronic pulmonary diseases such as asthma and bronchiectasis may also predispose to Klebsiella infections. The incubation period is short. Cough, pleuritic pain, and fever are frequent initial symptoms, with malaise, chills, and shortness of breath also occurring. The mortality rate is high, estimated at 70% to 80%. Of the remainder, a few recover slowly, whereas others suffer chronic disease with a clinical course similar to TB.

Radiographs most commonly show scattered lobular foci of airspace disease that may coalesce to form larger opacities (18,19). Typically, opacity has a sharp advancing border. This pattern is indistinguishable from other bacterial pneumonias. The upper lobes are involved in two-thirds of cases. Healing generally leaves residual scarring and distortion of lung. Radiographs may show expansion of a lobe (Fig. 5.12) due to “drowned lung.” Shift of the mediastinum to the opposite side may occur with whole lung involvement. Small pleural effusions are thought to occur because pleural thickening often remains after resolution of acute changes. Empyema occurs in about 6%. These patients have the worst clinical course. Klebsiella deserves consideration whenever opaque, extensive, expansive airspace disease is seen.

This gram-negative organism is a rare cause of CAP in healthy persons (20,21). Pseudomonas colonizes bronchiectatic airways, especially in cystic fibrosis. It is sometimes present on normal skin and is also a secondary contaminant in wounds. Prematurity, chemotherapy, antibiotics, steroids, immunosuppressants, old age, and debility increase the risk of Pseudomonas. Neutropenia increases the risk of severe disease. Most cases occur in hospitalized patients, especially those on ventilators. Organisms have been cultured from sinks, catheters, receptacles, ventilator equipment, and staff. Pseudomonas is the most common pathogen isolated from the lower respiratory tract of ventilated patients and is implicated in 25% of ventilator-associated pneumonias. Mortality in these patients is in
the 80% to 100% range. Pneumonia is usually secondary to aspiration of oropharyngeal contents in intensive care unit patients. Sedation, endotracheal intubation, and intermittent positive pressure ventilation predispose to infection. Person-to-person contact has been implicated in hospitals.