Small-airway diseases of the lungs: findings at expiratory CT.

CT performed in patients during suspended full expiration has recently been used to reveal a major physiologic consequence of airway diseases, particularly diseases of those smaller airways beyond the segmental branches: air trapping. Lung regions that retain air during exhalation (air trapping) remain more lucent than normal lung regions do. Detection of air trapping can provide clues to an otherwise unsuspected or underappreciated small-airway disease.