Pneumonia=Acute Pulmonary Infiltrate

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PA and Lateral films of RUL pneumonia

These are PA and lateral films of RML pneumonia (arrows). Note the indistinct borders, air bronchograms, and silhouetting of the right heart border.

Pneumonia is airspace disease and consolidation. The air spaces are filled with bacteria or other microorganisms and pus. Other causes of airspace filling not distinguishable radiographically would be fluid (inflammatory), cells (cancer), protein (alveolar proteinosis) and blood (pulmonary hemorrhage), Pneumonia is NOT associated with volume loss. Pneumonia is caused by bacteria, viruses, mycoplasmae and fungi.
The x-ray findings of pneumonia are airspace opacity, lobar consolidation, or interstitial opacities. There is usually considerable overlap. Again, pneumonias is a space occupying lesion without volume loss. What differentiates it from a mass? Masses are generally more well-defined. Pneumonia may have an associated parapneumonic effusion.
The type of pneumonia is sometimes characteristic on chest x-ray:
Lobar - classically Pneumococcal pneumonia, entire lobe consolidated and air bronchograms common
Lobular - often Staphlococcus, multifocal, patchy, sometimes without air bronchograms
Interstitial - Viral or Mycoplasma; latter starts perihilar and can become confluent and/or patchy as disease progresses, no air bronchograms
Aspiration pneumonia - follows gravitational flow of aspirated contents; impaired consciousness, post anesthesia, common in alcoholics, debilitated, demented pts; anaerobic (Bacteroides and Fusobacterium)
Diffuse pulmonary infections - community acquired (Mycoplasma, resolves spontaneoulsy) nosocomial (Pseudomonas, debilitated, mechanical vent pts, high mortality rate, patchy opacities, cavitation, ill-defined nodular) immunocompromised host(bacterial, fungal, PCP)