Figure 4.

FRI images of 2 patients displaying the changes in airway volume. The scale represents the percent change in the volume of the different airway branches at the different lung levels. Green represents an improvement, while red represents a worsening. The airway volume is the volume of the lumen of the airways and does thus represent the volume of air inside the airways. The airways are measured starting from the trachea at the top of the sternum up to the point where no distinction can be made between the intraluminal and alveolar air. This is where the airway diameter is around 1–2 mm. One can infer that an increase in airway volume means bronchodilation has occurred.¹⁸

Abbreviations: FRC, functional residual capacity; FRI, functional respiratory imaging; TLC, total lung capacity.