Plain Radiograph:
The retention of excess gas in all or part o fthe lung at any stage of expiration; to be used only when excess air is demonstrated by a dynamic study, such as inspiration/expiration films or fluoroscopy. Not to be used for overinflation
CT:
Decreased attenuation of pulmonary parenchyma, especially manifest as less than normal increase in attenuation during expiration. To be differentiated from decreased attenuation of hypoperfusion secondary to locally increased pulmonary arterial resistance.
The inspiratory and expiratory images shown below demonstrate air-trapping in the left lower lobe secondary to obliterative/constrictive bronchiolitis
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