Up to 75 ml of pleural fluid can collect in a subpulmonic location, before it is radiographically apparent on the lateral radiograph. Up to 175 ml of fluid is needed before it is radiographically apparent of the PA radiograph. On both views, an effusion is identified by "blunting" of the normally sharply triangular costophrenic margin. On decubitus views as little as 10 ml of fluid can be identified as it layers between the rib margin and the lung. If the patient is supine when a frontal view is obtained, the fluid may only be appreciated as a hazy increase in density of the involved hemithorax. Because a large effusion can invert the diaphragm, it is difficult to quantify a right-sided effusion. If there is air in the stomach (the stomach bubble) a left-sided effusion can be quantified. Decubitus views are helpful in quantifying the effusion and documenting that it is free-flowing. Loculated effusion can appear mass-like. Occasionally, fluid may stay in a subpulmonic location and cause the appearance of an elevated hemidiaphragm. An ipsilateral decubitus view will document that fluid is present. Sometimes fluid will collect in the major or minor fissures, causing the appearance of a "pseudotumor".
 C49,
 C50
|
Comparison |
 C51,
 C52
|
Pleural effusion |
 C53,
 C54
|
Elevated right lung base due to subpulmonic effusion |