When fluid accumulates in the lungs, it may predominate in the alveolar (airspace) compartment or the interstitial compartment. Typical bacterial pneumonia is an accumulation of exudative material in the alveolar spaces . In addition to increasing the lung density, the infiltrate cancels the contrast between vessels and lung boundaries, and these structures disappear. Air filled bronchi, normally invisible, will be contrasted by infiltrate creating air bronchograms .