Compensatory emphysema is a normal physiological process, because the total area of the lung decreases after partial lung resection, and in order to ensure the body’s oxygen needs, the other side of the lung becomes compensated for the expansion, thus forming compensatory emphysema. Generally, compensated emphysema is asymptomatic. However, if the lung loses this function, it is non-compensated emphysema. The radiographic appearance of obstructive pulmonary atelectasis is related to the location of the obstruction and the presence of pre-existing lesions or co-infections in the atelectasis. The obstruction can be in the main bronchus, lobar or segmental bronchus, or fine bronchus, and result in one-sided, lobar, segmental, and lobular atelectasis. The extent of pulmonary atelectasis varies, as does its radiographic presentation. The radiographic appearance of obstructive pulmonary atelectasis is related to the location of the obstruction and the presence of pre-existing lesions or co-infections in the atelectatic lung. The obstruction can be in the main bronchus, lobar or segmental bronchus, or fine bronchus, resulting in a one-sided, lobar, segmental, and lobular atelectasis. The extent of pulmonary atelectasis varies, as do its x-ray manifestations. 1, one-sided pulmonary atelectasis X-ray is uniformly dense in the affected lung field, the mediastinum is shifted to the affected side, and the rib space is narrowed. The healthy side of the lung may have compensatory emphysema. Left-sided bronchial obstruction causes left-sided total pulmonary atelectasis, showing uniformly dense left-sided lung fields, mediastinal displacement to the affected side, narrowing of the intercostal space, and elevation of the diaphragm. 2. Lobar atelectasis The x-ray manifestations of different lobar atelectasis are different, but their common features are lobe shrinkage, uniform increase in density, and centripetal displacement of interlobular fissures. The mediastinum and hilum may have different degrees of displacement toward the affected area. Compensatory emphysema may be present in the adjacent lobes. (1) Upper lobe of the right lung is not dilated: the posterior anterior position shows a fan-shaped narrowing of the upper lobe of the right lung with increased density and a lateral upward shift of the horizontal fissure. When the upper lobe is mildly constricted, the horizontal fissure is curved in a concave downward direction, and when it is significantly constricted, the upper lobe may appear as a triangular dense shadow next to the mediastinum. The hilum is elevated, and even the upper half of the hilum disappears. The lung texture in the middle and lower lobes is upwardly displaced and sparse, and there may be compensatory emphysema. The trachea may shift to the right. (2) Right middle lobe atelectasis: it is more common, and the posteroanterior view shows a dense lamellar shadow with a clear upper border and a blurred lower border in the right lower lung field against the right edge of the heart, and the right edge of the heart cannot be distinguished. In the lateral view, it appears as a triangular dense shadow with a band or tip pointing anteriorly and inferiorly from the pulmonary hilum. There may be compensatory emphysema in the upper and lower lobes. (3) Left upper lobe of the lung (including the lingual segment) is not dilated: the upper lobe of the left lung is thicker in the upper part and thinner in the lower part. When the lung is not dilated, the posterior anterior position shows a lamellar blurred shadow of the upper and middle lung fields of the left lung, with higher density in the upper part and lighter density in the lower part, unclear borders, left shift of the trachea, unclear left edge of the heart, and the whole oblique fissure is seen to be displaced forward in the lateral position, and the lobe of the non-dilated lung is high in density and shrunken. The lower lobe shows compensatory emphysema while the apical part of the lower lobe expands upward to the level of the 2nd thoracic vertebra. (4) Lower lobe atelectasis: Both sides of the lower lobe atelectasis showed a triangular dense shadow with the tip of the lower lung field on top and the base on the bottom, and the hilum shifted downward, sometimes the lower part of the hilum disappeared. There was compensatory emphysema in the upper and middle lobes. Lower lobe opacification of the right lung is more clearly shown than the left because of the overlapping cardiac shadow on the left side, but it can be shown on oblique or overexposure films. In lateral position, the lower lobe opacification shows oblique fissure shifted posteriorly and inferiorly with high density in the lower lobe.