Expiratory HRCT is an useful method for detecting small airway obstructive disease in which the air remain trapped in the small airways even after the expiration and appear as mosaic attenuation.
Ideally, an expiratory HRCT scan should be performed in all obstructive airway diseases including:
- obliterative bronchiolitis
- cryptogenic organising pneumonia
- bronchial asthma
- hypersensitivity pneumonitis
The static or dynamic images are taken during the breath hold after the forced expiration. The quantitative analysis is done by relative difference in density (HU) in inspiratory and expiratory scans.