Pleural effusion is caused by rapid formation and slow resorption of fluids in the pleural cavity. Infection, tumor, injury, autoimmune disease, heart failure, hypoproteinemia, and radiotherapy can cause pleural effusion. Pleural effusion is divided into exudation and transudation, and can be clear, purulent, bloody, chylous, and cholesteric.
X-ray
The manifestations of pleural effusion are related to the amount, position, and encapsulation or adhesion, and can be divided into free pleural effusion and localized pleural effusion.
Free pleural effusion
Small amounts of effusion
In a standing position, very small amounts of effusion accumulate at the lowest posterior costophrenic angle, and only the lateral view shows that the posterior costophrenic angle is blunt. When the amount of effusion reaches about 250ml, the posteroanterior chest radiograph shows that the lateral costophrenic angle is blunt and shallow. As the amount of effusion increases, the lateral costophrenic angle is invisible; the dome of diaphragm covered by effusion shows arcuate dense opacity with hyperdense outside and hypodense inside, and the upper edge is below the front end of the 4th rib. In a supine position, due to the dispersion of the fluid, pleural effusion is not easy to show, and the posteroanterior chest radiograph only shows hyperdense lung fields or interlobar fissure thickening.
Medium amounts of effusion
The upper edge presents an arcuate concave surface, and the middle and lower lung fields on the affected side show homogeneous dense opacities. The upper boundary exceeds the lower edge of the front end of the 4th rib and is below the plane of the lower edge of the front end of the 2nd rib.
Large amounts of effusion
The upper edge of the arcuate concave surface exceeds the lower edge of the front end of the 2nd rib, and the lung field on the affected side presents homogeneous dense opacity, and sometimes only the apex of the lung is lucent. The intercostal space on the affected side widens, the diaphragm descends, and the mediastinum shifts toward the healthy side.
Localized pleural effusion
Localized pleural effusion can be divided into encapsulated effusion, interlobar effusion, and subpulmonary effusion.
Encapsulated effusion
Encapsulated effusion is effusion confined to a certain part of the pleural cavity resulting from adhesion of the visceral and parietal pleura, mostly in pleurisy, mainly in the posterior lateral chest wall of the lower chest. On the tangential film, encapsulated effusion shows semicircular, sharply marginated, homogeneous, dense opacity protruding from the chest wall to the lung field, and its upper and lower edges intersecting the chest wall at an obtuse angle.
Interlobar effusion
Interlobar effusion is effusion confined to the horizontal fissure or oblique fissure, can exist alone or coexist with free effusion. Effusion in the oblique fissure is often difficult to diagnose on the frontal chest film, but is easy to find on the lateral chest film. The typical manifestation is homogeneous, sharply marginated, fusiform opacity in the interlobar fissure. Free effusion entering the interlobar fissure is often confined to the lower part of the oblique fissure, and lateral chest film shows triangular opacity, with increased density, with the tip pointing backward and upward.
Subpulmonary effusion
Subpulmonary effusion is pleural effusion between the lung base and the diaphragm, mostly on the right side. The lower edge of the lung pushed upward by the subpulmonary effusion is dome-shaped, which is easily mistaken for diaphragm elevation. The highest point of the dome caused by subpulmonary effusion is at the outer 1/3, and the costophrenic angle is deep and sharp. Supine chest radiograph can show the diaphragm in the normal position, which can be used for identification.
Figure 1 Pleural effusion
a. Plain chest film shows medium amounts of pleural effusion (↗) on the left side of the chest; b. CT mediastinal window shows small amounts of pleural effusion (↗) on the right side of the chest.
CT
Small or medium amounts of free effusion are manifested by narrow arcuate or crescent opacity of liquid density under the posterior chest wall, with smooth edges. Prone examination shows that the liquid has moved to the anterior chest wall.
Large amounts of free effusion show entire chest cavity is occupied by opacity of liquid density, the lungs are compressed to the hilum and presents opacity of soft tissue density, and the mediastinum is displaced to the opposite side.
Encapsulated effusion is manifested by arcuate opacity of liquid density protruding from the chest wall to the lung field. The base is wide and close to the chest wall. The angle with the chest wall is mostly obtuse, the edge is smooth, and the adjacent pleura is often thickened, forming pleural tail sign.
Interlobar effusion is manifested by zonate opacity of liquid density at the interlobar fissure, sometimes fusiform or spherical. When the amount of effusion is large, it may be lumpy and easily mistaken for lung tumor. Its location and running are consistent with the interlobar fissure, and there is no enhancement on contrast-enhanced CT, which can be used for identification.
Figure 2 Interlobar and encapsulated effusion
a. CT Pulmonary window; b. CT mediastinal window. The opacity of fluid density (↗) running along the oblique fissure in the right lung field is interlobar effusion; the semicircular opacity of fluid density (↗) protruding from the left chest wall to the lung field is encapsulated effusion.
MRI
Pleural effusion is mostly hypointense on T1WI, effusion rich in protein or cellular components is isointense or hyperintense, and bloody pleural effusion may be hyperintense. All kinds of pleural effusion are hyperintense on T2WI.