Comparison between a normal costophrenic angle on the patient's right, and an obscured costophrenic angle (circled) on the patient's left, due to pneumonia
with parapneumonic effusion
In anatomy, the costophrenic angles are the places where the diaphragm (-phrenic) meet the ribs (costo-).
Each costophrenic angle can normally be seen as on chest x-ray as a sharply-pointed, downward indentation (dark) between each hemi-diaphragm (white) and the adjacent chest wall (white). A small portion of each lung normally reaches into the costophrenic angle. The normal angle usually measures thirty degrees.
With pleural effusion, fluid often builds up in the costophrenic angle (due to gravity). This can push the lung upwards, resulting in "blunting" of costophrenic angle. The posterior angle is the deepest. Obtuse angulation is sign of disease