Q. How can you describe Pleural Effusion?
Fluid has a density indistinguishable from soft tissue on a radiograph. Pleural fluid tends to accumulate in the deepest part of the posterior costophrenic angle. Small effusions are thus identified earlier on a lateral view. Ultrasonography is also capable of picking up very small effusions.
An effusion may not be recognized on a PA view until 100 to 200 ml of fluid has accumulated and has caused blunting of the costophrenic angle. Larger effusions have a fairly well defined concave upper edge (which is higher laterally than medially) and obscure the diaphragm, and later the mediastinal borders.