Cavity is formed after the necrotic tissue in the lung is discharged through aerated bronchi. The cavity wall can be necrotic tissue, granulation tissue, fibrous tissue, and tumor tissue, predominantly in pulmonary tuberculosis, lung cancer, and fungal diseases. According to the thickness of cavity wall, cavity can be divided into thick-walled cavity and thin-walled cavity. The thickness of the former is ≥ 3mm, and the thickness of the latter is < 3mm. Different from cavity, intrapulmonary air containing space is pathological enlargement of physiological cavity in the lung. Pulmonary bullae, air-containing lung cysts, and pneumatocele have intrapulmonary air containing spaces.
X-ray
Thin-walled cavity
The cavity wall can be a thin layer of fibrous tissue, granulation tissue, and caseous tissue. The cavity is round, oval, or irregular; the inner and outer edges of the cavity wall are smooth and clear; and there is mostly no fluid level in the cavity. There is no large, peripheral, patchy opacity, but there may be punctate lesions. Cavity is common in pulmonary tuberculosis, and sometimes lung metastatic tumor can also present thin-walled cavities.
Figure 1 Cavity and intrapulmonary air containing space
a. Chest plain film shows intrapulmonary air containing space in the right lung field (↗); b. on CT lung window, the space (↗) has an irregular morphology, the inner edge of the space wall is rough, and the outer edge is lobulated, it is a cancerous space.
Thick-walled cavity
The thickness of cavity wall is often more than 3mm, mostly more than 5mm. Cavity is surrounded by hyperdense consolidation, and the inner wall is smooth or rough, and is more common in pulmonary tuberculosis and peripheral lung cancer. The outer edge of tuberculous cavity wall is mostly neat and clear, and there are often peripheral scattered patchy lesions. The outer edge and contour of the cavity wall of peripheral lung cancer are lobulated and accompanied by fine short spicules, the inner surface of the cavity wall is rough, and sometimes nodules on the wall can be seen.
Intrapulmonary air containing space
The wall of the intrapulmonary air containing space is thin and homogeneous, with a thickness of less than 1mm, no peripheral consolidation, and no liquid in the space. When complicated by infection, an air-fluid level can be seen in the space, and there may be peripheral consolidation.
CT
Compared with chest x-ray, CT can detect cavity in the lesion more sensitively and show its details.
Cavity location
Tuberculous cavities are often seen in the apical segment and posterior segment of the upper lobe or dorsal segment of the lower lobe. Cancerous cavities are mostly in the anterior segment of the upper lobe and the basal segment of the lower lobe.
Cavity size
Cavities with a diameter greater than 3 cm are mostly tumor.
Cavity wall
If the outer edge of cavity wall is irregular or lobulated, and the inner edge is rough or nodular, they are mostly cancerous cavities. The thickness of cavity wall less than 4mm suggests mainly benign lesions, while the thickness greater than 15mm is suggestive of mostly malignant lesions.
Peripheral pulmonary tissue
Irregular linear, nodular, or patchy satellite lesions and thickening of bronchial walls connected to the hilum of the lung can be seen around tuberculous cavities. Bronchial stenosis or obstruction can sometimes be seen in cancerous cavities, and signs of obstructive pneumonia can be seen.
In intrapulmonary air containing space, the cyst wall of congenital lung cysts is mostly thin and homogeneous, with a thickness of about 1 mm. The wall of pulmonary bullae is thinner than that of the congenital air-containing lung cysts, less than 1 mm, and homogeneous. Pulmonary bullae often occur in the subpleural area, and the size varies greatly, generally small, and the large ones can occupy a whole lung lobe.
MRI
On T1WI and T2WI, cavity is hypointense due to the presence of air, and the cavity wall is isointense. However, due to the low spatial resolution, MRI is not as good as CT in the presentation of detailed cavity wall.