Presentation
Emergency admission with a few days cough, mild fever and malaise
Patient Data
Admission radiograph
Scoliosis
Right heart border obscured
Subtle ill-defined increased opacity in the right lower zone
Horizontal fissure not visible
There is little or no displacement of the mediastinum to the right: the minor volume loss is easily compensated for by expansion of the right upper and lower lobes.
Right middle lobe collapse and consolidation bounded by a depressed horizontal fissure and a bowed oblique fissure.
Consolidated right middle lobe obscures the right heart border, (silhouette sign). Interfaces between structures are best seen when there is a marked difference in attenuation.
On CT, boundaries that are perpendicular to the reformat plane appear sharp. With increasing obliquity, margins appear less sharp, especially as slice width is increased. The oblique fissure is more blurred than the horizontal fissure. The blurring is due to volume averaging.
The orientation of the fissures explains why the boundaries of the consolidated right middle lobe are not visible on the PA CXR: they are not tangential to the X-ray beam. Lateral CXR would clearly demonstrate the sharply outlined RML, but they are not routinely performed.
3 days later following...
3 days later following bronchoscopy
At bronchoscopy, a mucus plug was cleared from the right middle lobe bronchus. The following day, an X-ray showed a clearing of the right middle lobe and a re-appearance of the right heart border.
The horizontal fissure is now visible as a thin line in a normal position, intersecting the 6th right rib at the lateral chest wall. There is little or no change in the position of the heart.
Case Discussion
The patient tested positive only for rhinovirus. Bronchoscopy demonstrated a mucus plug in the right middle lobe bronchus and no tumor. Follow-up CXR the next day was normal.
The signs of right middle lobe collapse and consolidation are subtle. As a differential diagnosis in an asymptomatic patient consider pectus excavatum which also causes poor definition of the right heart border and increased opacity in the right lower zone. However, in pectus excavatum, the heart would be displaced to the left, and the anterior ribs would be more vertically oriented.
Radiographic interpretation requires a thorough examination of the image and a thorough knowledge of normal appearances. The CXR is a puzzle to be solved. Everything between the anode and the detector is superimposed on the image. This is the ultimate in volume averaging, and the degree of blurring or clarity depends on the orientation of interfaces with respect to the X-ray beam.
Lastly, note how subtle the lower zone opacity is on X-ray compared to the CT scan which benefits from superior contrast resolution and minimal volume averaging.