On imaging a cavity is an air-containing cavity with an intact wall, which is usually 1 mm or more. Cavities are a common imaging manifestation of lung disease. Many diseases can form cavities in the course of development, and the etiology and morphology of cavities in different diseases have their own characteristics. CT examinations are more accurate than radiographs in determining the presence of cavities and in making a qualitative diagnosis. High-resolution CT (HRCT) can further reveal the subtle manifestations of cavities and provide more imaging information. Pathologically, cavities are formed when the lesion is necrotic and liquefied and air is introduced through the bronchial tubes. Those that do not introduce gas into the lesion are not referred to as cavities, but rather as necrosis or abscesses. For neoplastic lesions, the central part of the lesion is necrosis and liquefaction of the tumor tissue, which can be combined with infection after the lesion is connected to the outside world. For inflammatory diseases the cavity is a septic lesion of the lung or a tuberculous caseous lesion. The wall of the cavity retains the pathological features of the original lesion. Cavities are divided into single and multiple cavities according to their number, and into intrapulmonary cavities and intra-pulmonary solid cavities according to their morphology. Single cavity (a) single cavity lesion in the lung: 1, peripheral type bronchial lung cancer: the incidence of cavity in peripheral type lung cancer is 2-16%, of which: squamous cell carcinoma accounts for 80%, adenocarcinoma and large cell carcinoma accounts for 20%, small cell undifferentiated carcinoma generally does not occur cavity. 2.Tuberculosis: cavities account for about 40% of pulmonary tuberculosis in adults. It is mainly seen in secondary tuberculosis, but a few primary lesions can also form cavities. Cavities may be thick-walled or thin-walled. Cavities in pulmonary tuberculosis are divided into: ① Cavities in infiltrative caseous foci: cavities resulting from caseous necrosis within infiltrative lesions. The cavity wall is thin and consists mainly of proliferating tuberculous granulation tissue with thin caseous material on the inner wall. ② Cheese cavities, or fibrous cheese cavities, are cavities arising from tuberculosis spheres or cheese lesions with a thicker cheese layer and thinner tuberculous granulation tissue and fibrous envelope. The fibrous envelope of the nodule bulb is intact. (iii) Fibrous cavity: with a typical three-layer structure of caseous necrosis, tuberculous granulation tissue, and fibrous tissue. Fibrous tissue is the main component of the cavity wall. Due to the contraction and pulling of the fibrous tissue, the cavity shape is irregular. 3.Lung abscess: the wall of acute lung abscess is mainly inflammatory exudative lesion, and the wall of chronic lung abscess is mainly composed of fibrous weave. Lung abscess occurs after pneumonia, inhalation and by extra-pulmonary spread, the latter seen in amoebic lung abscess. 4, pulmonary mycosis: domestic reports are mainly seen in new Cryptococcus and Aspergillus. 5, pneumoconiosis cavities: cavities occur on the basis of progressive pneumoconiosis fusion blocks, often combined with pulmonary tuberculosis. Cavitation is mainly formed by the liquefaction and discharge of caseous material from the lesion. Cavernous lesions are large, irregular in shape, and the cavity walls are predominantly thick-walled with uneven thickness. 6.Other diseases: pulmonary infarction, nodular disease. A group of 159 cases of nodular disease were reported to have cavities in 3 cases. (B) Differential diagnosis of single cavity in the lung The differential diagnosis of single cavity is based on the size of the cavity lesion, the thickness of the cavity wall, the performance of the inner and outer edges of the cavity, and the abnormal morphology in and around the cavity. 1. size of cavernous lesion: cavities occurring in nodules below 2 cm are more common in tuberculosis and rarely in lung cancer below 2 cm. cavities occurring in masses above 4 cm are more common in lung cancer. Some tuberculosis cavities such as fibrous thick-walled cavities and fibrous cheese cavities are also larger, the former with irregular morphology, and some of the latter are difficult to differentiate from lung cancer and need to be combined with clinical and laboratory tests. Chronic lung abscess cavities can be larger or smaller. Coal workers’ pneumoconiosis cavity lesions are larger. 2, the thickness of the cavity wall: generally the cavity wall on 3mm is called thick-walled cavity, 3mm below the thin-walled cavity. Thick-walled cavities are more common in lung cancer, tuberculosis, fibrous caseous cavities, fibrous thick-walled cavities, acute and chronic lung abscesses. Thin-walled cavities are seen in infiltrating caseous focal cavities and fibrous thin-walled cavities of pulmonary tuberculosis. Uneven thickness of cavity walls is seen in lung cancer and tuberculosis. Significant thickness unevenness makes the cavity eccentric or peculiarly shaped. The walls of lung cancer cavities are thicker on the hilar side, and the cavity is eccentric on the lateral side. The caseous lesions of the tuberculosis bulb at the opening of the draining bronchus are the first to soften, so that the cavity lumen begins mostly on the hilar side of the lesion, where it joins the draining bronchus, and is small and round. The cavity develops further in the form of a crescent, which is also located on the hilar side of the lesion. Finally, it can form a round-like cavity. The cavity caused by mycobacteria can be thick-walled, thin-walled or of uneven thickness depending on the type of pathogenic bacteria. 3, the inner edge of the cavity: the inner edge is smooth in lung abscess, tuberculosis fiber cavity. The inner edge is rough in lung abscess and fibrous cheese cavity of pulmonary tuberculosis. The uneven inner edge of the cavity is seen in lung cancer and tuberculosis fibrous case cavity. The wall nodules on the inner edge of the cavity mainly occur in lung cancer, and the unliquefied cheese material in the fibrous cheese cavity of pulmonary tuberculosis can also form wall nodules. 4.The outer edge of the cavity: the outer edge of the cavity is clear in tuberculosis fibrous case cavity and chronic lung abscess, and the outer edge of some lung cancer cavities is also smooth and clear. The outer edge of the cavity with burr and “radiating crown” image is seen in the fibrous thick-walled cavity of pulmonary tuberculosis and lung cancer. Lobes on the outer edge are seen in lung cancer and chronic lung abscess. 5, around the cavity: satellite foci are seen in various cavities of pulmonary tuberculosis. Linear images between the lesion and the pleura can be seen in lung cancer, tuberculosis and lung abscess cavities. A lamellar infiltrative image around the cavity is seen in acute lung abscesses, infiltrative caseous focal cavities, and a limited lamellar image is seen near chronic lung abscesses. Pneumothorax and fibrous cords around the cavity are mostly seen in pneumoconiosis. 6, cavity cavity contents: gas-fluid flat is mainly seen in acute lung abscess. It is generally believed that tuberculosis cavities have no gas-fluid surface, but some studies point out that gas-fluid surface accounts for 9%-21% in tuberculosis cavities. It is mostly co-infection and hemorrhage. The solid components of the cavity are tumor nodules, caseous necrotic material, clots, and mycobacterial balls, etc., which make the cavity appear in different forms against the gas in the cavity. Mycobacterial balls occur in lung cancer, tuberculosis and chronic lung abscess cavities, or within bronchiectasis and pulmonary cysts, as round-like movable nodules, mostly located at the site of fallout. Crescent-shaped cavities are bow-shaped gas shadows, always located above the mycobacterial sphere. If the cavity contents are attached to the cavity wall, such as invasive mycosis, lung cancer, tuberculosis cavity, crescent-shaped gas shadow can be located lateral or inferior to the cavity. If the cavity contents are attached to the anterior or posterior wall of the cavity, a “target-like sign” is formed in the posterior-anterior projection. Some solid contents are located above the liquid to form the “water floating sign”, which is seen after the endothelial rupture of fine-grained echinococcosis cyst, which belongs to the cavity lesion and should be differentiated from the cavity lesion. 7, CT enhancement performance: generally used for the differential diagnosis of thick-walled cavities around 2~3cm. The cavity wall of fibrous cheese cavity of pulmonary tuberculosis does not strengthen or has thin peripheral strengthening, while the wall of lung cancer cavity is largely strengthened. Second, multiple cavities (b) lesions of multiple cavities in the lung: 1, pulmonary tuberculosis: any tuberculosis cavity can be multiple, mostly bronchially disseminated tuberculosis cavities, and hematogenous disseminated tuberculosis can also form multiple cavities. 2, pulmonary metastases: cavities can occur in pulmonary metastases of any primary tumor, and cavities in pulmonary metastatic nodules account for about 4%,. Squamous epithelial carcinoma is the most common, accounting for 69% of cavernous thoracic metastases in X-ray examination. However, according to CT examination, cavitation has been reported in 9.5% of adenocarcinoma metastases and 10% of squamous carcinomas. The sites of primary malignant tumors that have been reported to have cavitary lung metastases are: head and neck, thyroid, breast, bone, kidney, pancreas, colon and rectum, bladder, penis, testis and uterus. The cause of cavities is necrosis due to insufficient blood supply. The wall of the cavity is irregularly thick-walled to very thin and smooth. Thin-walled metastatic cavities are mostly caused by the primary sarcoma. 3.Hematogenous multiple lung abscesses: caused by right Staphylococcus aureus pendulum hemorrhage. 4.Mycobacteria: mainly seen in cryptococcal infections. 5, other diseases: parasitic diseases (mainly seen in pulmonary schistosomes), collagen-vascular diseases (granuloma of Weil, rheumatoid nodules, etc.), sarcoidosis (eosinophilic granuloma,), vascular diseases (septic emboli, mostly caused by trauma or vascular intrinsic retained catheters, causing multiple small vessel embolism and septic inflammation and cavities.) , malignant lymphoma, nodal disease and histiocytosis X, etc. (B) Differential diagnosis of multiple cavities The differential diagnosis of multiple cavities in the lungs should be combined with the distribution characteristics of the cavities, the location of the lesions, the combined images and dynamic changes in the lungs, etc. 1, multiple smaller cavities in both lungs: cavity lesions are mostly below 2 cm. They are mainly seen in pulmonary tuberculosis, pulmonary metastases and pulmonary abscesses. Less common diseases are eosinophilic granuloma, septic pulmonary embolism/pulmonary infarction, etc. The differential diagnosis is based on the morphology of the cavity and the combined lesions in the lungs. (1) Pulmonary tuberculosis: cavities are uneven in size and can be thin-walled or thick-walled. In the differential diagnosis, each cavity should be noted and generally has the characteristics of a solitary tuberculous cavity. If the cavity is lateralized to the hilar side of the lung, there are draining bronchi, surrounded by satellite foci, combined with speckled and corrugated images elsewhere in the lung, the lesion has uneven density and may have foci of calcification. The lesions are more frequent in the posterior segment of the two lung apices and the dorsal segment of the lower lobe. (2) Metastases: Multiple cavities in the lungs are often combined with multiple nodules. The overall distribution of cavities and nodules is characterized by a random distribution of nodules, i.e., located in the subpleural, peri-bronchial vascular bundles and lung parenchyma, and the density of distribution is approximately the same in all areas. The size of the lesions varies, and the density of the lesions is more uniform. (3) Multiple lung abscesses: cavities of uniform or non-uniform size, cavity walls are mostly thicker, fluid levels may be present in the cavity, and multiple patches and faint nodular lesions combined in the lung are more common. (4) Other diseases: Eosinophilic granuloma: granulomatous lesions dominated by eosinophils around the fine bronchi, forming multiple small nodules and cavities within the nodules, with lesions distributed in the center of the lobules and more common in the upper lobes. Septic pulmonary embolism/pulmonary infarction: multiple nodules and cavities in the lungs, located in the periphery of the lungs and under the pleura. Some of the lesions are connected to blood supply vessels. 2. Multiple large cavities scattered in both lungs: tuberculosis is the most common. (1) Tuberculosis: can be cavities with infiltrative cheese foci, fibromatous cavities and fibrous thick-walled cavities surrounded by spots, nodules and cords images, mostly located in the post-apical segment of the upper lobe and the dorsal segment of the lower lobe. (2) Mycobacteria: newer cryptococci are more common, with blurred outer edges of cavities, combined with lamellar and blurred nodular images, and rapid dynamic changes. (3) Pulmonary schistosomiasis: generally thin-walled, single- or multi-housed, and may be surrounded by striae and patchy images. (4) Wechsler’s granulomatosis: the main lesion is a multifocal nodule in the lung, consisting of granulomas and inflammation. Cavitation occurs within the larger nodules, mostly occurring in lesions of 2 cm or more. (5) Lymphoma: Cavitation occurs in nodular and mass-type lymphomas. The lesions are multiple, vary in size, and are thin-walled or thick-walled cavities. (6) Vascular septic emboli: multiple cavitations combined with multiple nodules and wedge-shaped images. The cavities can be seen to be connected to the blood supplying vessels. C. Cavitation of lobar and segmental lesions Lobar and segmental solid lesions or pulmonary atelectasis can be combined with cavitation, mainly lobar pneumonia, pulmonary abscess, tuberculosis and lung cancer. (a) Pneumonia 1, acute pneumonia combined with lung abscess Some lobar pneumonia can be combined with acute lung abscess. x-ray and CT show transillumination and gas-fluid flat within the solid images of lobes or lung segments. The cavity is usually large. The main pathogenic organism is S. pneumoniae. Other diseases that cause similar images are certain Gram-negative bacilli, secondary lung abscesses, mostly in patients with immune impairment. 2, lung abscess combined with chronic pneumonia Chronic pneumonia may present with solid shadowing of lobes or segments of the lung, which may be combined with reduced lung volume. It is usually a single cavity and can be combined with bronchiectasis. (b) Tuberculosis 1, worm-like cavities: also known as wall-less cavities, cheese cavities, seen in case of caseous pneumonia and large foci of fibrous cheese. The cavity size is about 0.5cm-1.cm, round, and the cavity wall is blurred. 2.Sclerotic multifocal cavity: the cavities in the lungs destroyed by pulmonary tuberculosis are multiple, round or irregular, often closely connected, with a large amount of fibrous connective tissue in the cavity wall, surrounded by a variety of forms and densities of tuberculosis foci, such as cheese tissue, granulation tissue, pulmonary sclerosis, pleural thickening, etc. 3, chronic fibrous cavity type tuberculosis: the cavity is a fibrous thick-walled cavity, often multiple. Surrounded by infiltration, cheese nodules, fibrosis and pleural thickening and other lesions, the volume of the lung is reduced. 4.Lung abscess combined with obstructive bronchial disease Central type lung cancer or other tumors or lesions of bronchus caused by obstructive pneumonia and pulmonary atelectasis can be combined with lung abscess. CT enhancement scan or MRI can show necrotic and liquefied lesions in obstructive pneumonia. The cavity-like lesions in the lungs need to be differentiated from cavities. A cavity is an abnormal expansion of a physiological cavity in the lung. Common lesions include pulmonary cysts, pulmonary alveoli, and air sacs. On imaging, a cavity has a wall of 1 mm or less, which is the main basis for differentiation from a cavity. In the differentiation of cavities, isolated cavities are usually cysts, while cavities with emphysema are usually alveoli and air sacs are the combined manifestation of golden pluvial pneumonia. In some cases the cavity resembles a cavernous lesion and should be further differentiated. 1, pulmonary alveoli combined with infection: solid lung tissue around pulmonary alveoli, manifested as lamellar shadows or round translucent areas within solid lung shadows, or combined with fluid levels, similar to lung abscesses. In the differential diagnosis, it should be noted that pulmonary alveoli tend to occur in the apical, basal, and outer lung bands. There are images of pulmonary alveoli and emphysema around and contralateral to the lesion. It can be confirmed by reexamination after the absorption of inflammation. 2.Pulmonary cyst co-infection: mostly seen in children. Pulmonary cysts have thickened walls, fluid levels, and lamellar images around them. The diagnosis of pulmonary cyst can be confirmed after the inflammation is absorbed. 3.Pulmonary cyst malignancy: rarely seen. It is manifested as limited thickening and nodules on the thin wall of the pulmonary cyst. 4, pulmonary isolation disease: injection such as into the contrast agent routine CT examination shows that the blood supply artery accounted for 80%, spiral CT angiography has a better diagnostic effect. 5, Cysticercus carinii pneumonia: the incidence of cystic lesions in the lung is 10%-34%, and the cystic lesions are absorbed after treatment. 6, cystic lesions in the lung: lymphangioleiomyomatosis: multiple cystic lesions, 2cm~5cm in size, thin wall, diffuse distribution in the lung. Surrounding lung tissue is normal.