A small amount of effusion accumulates in a subpulmonic location, causing slight elevation of the hemidiaphragm.
As the fluid increases, the fluid starts to spill over into the most dependent costophrenic sulci.
Fluid accumulating posteriorly can be seen on the lateral view before it becomes visible on the frontal view.
When the fluid is slightly above the level of the upper portion of the diaphragm, blunting of the lateral costophrenic angle is seen. This is the earliest sign of pleural effusion on the frontal view.
A minimal amount of fluid (approximately 175 mL) is required to produce detectable blunting. As much as 500 mL of pleural fluid can be present without apparent changes on the frontal view.
A large free pleural effusion appears as a dependent opacity with lateral upward sloping of a meniscus-shaped contour.
The diaphragmatic contour is partially or completely obliterated, depending on the amount of the fluid (silhouette sign).
Because the fluid is laterally tangential to the x-ray beam, the depth of fluid penetration increases and consequently increases attenuation of the radiation. The depth of the fluid penetrated anteriorly and posteriorly is small, especially in the upper portion of the effusion.
A very large pleural effusion appears as an opaque hemithorax with a mediastinal shift to the contralateral side. The mediastinal shift can be less prominent or even absent in the presence of underlying lung pathology (eg, atelectasis) or contralateral hemithorax abnormality.