Infiltrate -- Superior Segment Right Lower Lobe

An airspace (alveolar filling) abnormality is present in the right lower lung field.
- The opacity is hazy, indistinct and air bronchograms are suggested.
Given the patient's clinical history of fever, pneumonia would be the leading differential diagnostic possibility.
But, where is the infiltrate?
- If it were in the medial segment of the RML, it should silhouette the right heart border. This is not the case here.
- We might be able to place the infiltrate in the RUL if we could locate the horizontal (minor) fissure.
  - Two, roughly parallel, lines are seen in the area.
  - The more inferiorly placed line, which appears to define the lower border of the infiltrate, should be too low to be the horizonal fissure
- The lateral film is diagnostic.
  - It clearly places the infiltrate posteriorly, in the superior segment of the right lower lobe, which on the PA film overlaps right middle lobe territory.
  - Overlap of territories on the PA film can lead to errors in localization of abnormality, and illustrates the need for a lateral (orthogonal view).

Ref: Armstrong, P., et. al., Imaging of Diseases of the Chest, 2nd ed, Mosby, St. Louis, MO, 1995, pp. 58-124.