Concern here was for an inhaled foreign body. A frontal radiograph demonstrates slight differential transradience between the left and right hemithorax.
Where there is uncertainty about the underlying cause, an expiratory film may be sought in patients who can cooperate. An 18 month old cannot and another solution needs to be considered.
Bilateral decubitus lateral films allows assessment of air-trapping caused by an inhaled foreign body. The expectation is that the dependent lung will collapse partially in the normal patient. Where there is an obstructive foreign body there will be air-trapping and hyperlucency of the dependent lung.
In this case, the lateral also highlighted the radiodense stone obstructing the left main bronchus which was barely visible on the frontal projection.