What appearance is indicative of pleural effusion on imaging?

The characteristic that indicates pleural effusion on imaging is the blunting or obliteration of the costophrenic angle. This phenomenon occurs because fluid in the pleural cavity accumulates and fills the potential space between the visceral and parietal pleura. In a normal imaging study, the costophrenic angles are sharp and well-defined; however, when fluid is present due to pleural effusion, this angle becomes blunted or obscured, as the fluid levels can be visualized.

Other imaging patterns can suggest different pathological conditions. Clear lung fields might imply that no major acute pathology is present, honeycomb patterns are typically associated with conditions like interstitial lung disease, and ground glass opacity reflects various lung processes including inflammation or edema but does not specifically indicate pleural effusion. Therefore, recognizing the blunting of the costophrenic angle is crucial in diagnosing pleural effusion through radiographic evaluation.