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| Oreo cookie. |
The classic sign of pericardial effusion on the lateral chest radiograph is the "Oreo cookie sign". The most anterior radiolucent line is the epicardial fat, the radiopaque line is the pericardial effusion, and the posterior radiolucent line is the pericardial fat.
Oreo cookie sign:
Epicardial fat and retrosternal fat stripes are the outer DARK cookie layers while the opaque fluid is the WHITE fluff of the cookie.