Most exams in the neonate will be performed in the supine position, so the air will rise to the least dependent portion of the body. The least dependent portion of the chest is the anterior, lower chest. This is where you should look for a pneumothorax, which will appear as an unusually sharp heart border or an unusually sharp and lucent costophrenic angle on a supine CXR. The hyperlucent costophrenic angle is known as the deep sulcus sign.

Right anterior pneumothorax demonstrating a sharp right heart border and a hyperlucent costophrenic angle.