Q) What are the radiological signs of pleural effusion?

A: A small amount of free fluid is usually undetectable on chest radiographs. It collects first under the lower lobe - such subpulmonic effusions are detected by lateral decubitus Chest radiographs. Later, there is blunting of posterior & lateral Costophrenic angles (200 - 500ml of effusion). Then there is homogenous opacification of lower chest.

An empyema is radiologically indistinguishable from an effusion on chest radiograph, but should be suspected if pleural effusion is large, delayed in appearance, distributed unusually or loculated.

Fissural location of free fluid appears sharply marginated, biconvex and has a tail along the fissure.

When in supine position, pleural effusion appears as a hazy opacity. Associated haziness of diaphragmatic margin, blunting of costophrenic angle, thickening of minor fissure & widening of paraspinal interface may be apparent.

Encysted effusion in minor fissure :