Appearance of Alveolar Patterns
The final stage in the development of pulmonary edema and the most advanced stage is represented by the radiographic finding of alveolar pulmonary patterns. The hallmark of this disease are air bronchograms and lobar signs.
Air bronchograms are formed when continued fluid accumulation occurs into the alveoli themselves, resulting in alveolar edema. When the alveoli accumulate fluid, they appear radiopaque and only the air filled bronchioles remain visible. As you can see from the picture (side), the air bronchograms are easily detectable against such an opaque, fluid-filled lung.
A lobar sign is present when a sharp line of demarcation is noted between a lung lobe that is opaque (consolidated) and adjacent to one that is either normally aerated or near normally aerated. Hence in a lobar sign, the margin of the consolidating process must extend to the margin of the lung lobe to provide the sharp radiographic contrast between the non-aerated and aerated lung lobes. Just like with fissure lines, one must ensure that this line of demarcation occurs where lung lobe fissures actually exist.
Lobar signs are seen when a patient has fluid accumulation in the alveoli and parenchyma and as a result, the affected lung lobe is radiopaque compared to its radiolucent adjacent lobes. The presence of a lobar sign indicates alveolar disease due to the presence of exudates, edema, and/or hemorrhage in that area.