What are the CT manifestations of alveolar microlithiasis?

① Small nodular calcification shadow: most of them are diffuse and symmetrical calcium particles distributed in both lungs, with a diameter of 1 ~ 2 mm, and the lesions are dense at the back of the lower lobe; ② Linear calcification shadow: the thin layer of calcium on the lung surface under the visceral pleura can be manifested as linear calcification shadow, which is more common in the anterolateral and middle lobe of the tongue segment; ③ Pulmonary bullae and pulmonary cysts: usually multiple, distributed under the pleura; ④ Reticular cord shadow: Reticular cord shadow may appear in the late stage due to pulmonary fibrosis.