Pneumothorax is caused by air entry in the pleural cavity due to rupture of the visceral or parietal pleura. Hydropneumothorax is caused by the presence of both fluids and air in the pleural cavity.

X-ray

There are no lung markings in pneumothorax, and chest x-ray shows opacity of air density. In mild pneumothorax, the involved area is linear or zonate area without markings, and the edge of the compressed lung can be seen and is clearly visible during exhalation. In severe pneumothorax, the involved area can occupy the middle and outer zones of the lung field, and the compressed lung is in the inner zone and shows homogeneous opacity of soft tissue density; the ipsilateral intercostal space widens, the diaphragm descends, and the mediastinum shifts to the healthy side. In hydropneumothorax, air-fluid level can be seen in the upright chest radiograph, and air-fluid level can traverse horizontally the entire chest cavity on the affected side in severe patients. If the visceral and parietal pleura are adhered, localized or multilocular pneumothorax or hydropneumothorax can be formed.

Figure 1 Pneumothorax
a. Chest plain film shows zonate lucent area without lung markings on the right side, and the edge of the compressed lung can be seen (↗); b. CT lung window shows extremely hypodense area without lung markings in the periphery of the right thoracic cavity, the right lung is compressed inward (↗).

CT

Pneumothorax is manifested by zonate extremely hypodense area without lung markings on the outside of the lung, and arcuate visceral pleura shows thin linear opacity of soft tissue density on the inside and is parallel to the chest wall; the lung tissue is compressed and collapsed, the entire lung is compressed to the hilum of the lung and is spherical in severe patients, the mediastinum shifts to the opposite side, and the diaphragm descends. In hydropneumothorax, due to gravity, the fluid is in the dorsal side and the air is in the ventral side, and clear air-fluid level and edges of collapsed lung can be seen.

MRI

MRI cannot show pneumothorax, and can only show fluid signal of hydropneumothorax.