Acute phase of hypersenstivity pneumonia (extrinsic allergic alveolitis)

CT scan shows ground glass opacification of the lungs during the acute phase of hypersensitivity pneumonia (extrinsic allergic alveolitis). Several secondary pulmonary lobules are spared and appear as darker areas in both lower lobes. Lower attenuation secondary pulmonary lobules in both lower lobes are indicative of gas-trapping. The resulting pattern has been dubbed "head-cheese sign."