Chest ImagingAcute bacterial pneumonia
Bacteria account for the significant majority of lung infections. Specific bacterial organisms may be implicated depending upon the background in which pneumonia occurs. For example, typical community-acquired disease is generally a result of infection with
Streptococcus pneumoniae,
Mycoplasma pneumoniae,
Legionella pneumophila,
Haemophilus influenzae, or
Chlamydia pneumoniae. Hospital-acquired infections are generally caused by Gram-negative organisms such as
Pseudomonas aeruginosa,
Klebsiella pneumoniae,
Escherichia coli, and
Staphylococcus aureus. In alcoholics and patients with altered mental status anaerobic bacteria may be responsible. Bacterial pneumonias are frequently life-threatening and it is important for the
radiologist to recognize the broad variety of appearances likely to be produced on chest radiographs by these infectious agents. Most typically community-acquired bacterial pneumonia produces a lobar homogeneous opacity in the lung periphery. More than one lobe may be involved (
Fig.1). The inflammatory response to these bacteria produces dense
consolidation because there is significant inflammatory fluid production which spreads rapidly through the pores of Kohn and the canals of Lambert into adjacent alveoli. The progress of this consolidation if unimpeded leads to the "classic"
lobar pneumonia. Some organisms, for instance anaerobes, have a tendency to occur in dependent portions of lung. Typical community-acquired bacteria tend to cause less lung destruction and cavitation than organisms associated with
bronchopneumonia and
anaerobe lung infection. Ipsilateral
pleural fluid collections are frequent in virtually all acute bacterial pneumonias, with the amount of fluid varying depending on the organism. Complications such as
empyema may lead to protracted recovery.
If untreated, acute bacterial pneumonias may spread throughout the lungs, subsequently resulting in systemic dissemination and death. If appropriately treated most bacterial pneumonias resolve quickly and signs of radiographic improvement should be evident within days with near or complete resolution expected within 2-3 weeks.
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A PA chest film demonstrates bibasilar areas of homogeneous opacification with some air bronchogram noted in both regions. This type of consolidation is very typical of bacterial infections such as S. pneumoniae, H. influenzae, and Klebsiella species.
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Acute bacterial pneumonia, Fig.1 | |