Hi!

Costophrenic angles should be neatly seen in X-Rays, but sometimes they look obliterated, often due to the presence of liquid (pleural effusion):

Here's a website:

"Small pleural effusions are suspected on CXR by blunting of the costophrenic angle on upright films, however, blunting may be absent on a supine film. Larger effusions are easily seen on PA and lateral films as obliteration of the costophrenic angle. The radioopacity is higher laterally (meniscus). Pleural effusions can also be located between the lung and the diaphragm. Such subpulmonic effusions are suspected by hemidiaphragm elevation with the apex of the "apparent" diaphragm displaced laterally and sloping toward the lateral costophrenic angle".

HTH.