Clinical meaning
Chest X-ray interpretation requires a systematic approach to avoid missed findings. The standard PA (posteroanterior) film is taken with the patient standing 6 feet from the X-ray source, with the anterior chest against the cassette, minimizing cardiac silhouette magnification. AP (anteroposterior) films, often portable, magnify the heart and mediastinum, making cardiomegaly assessment unreliable. X-ray images display five densities from most to least radiopaque: metal (white) > bone > soft tissue/fluid > fat > air (black). The systematic ABCDE approach covers: Airway (tracheal position, carina), Breathing (lung fields, pleural spaces), Cardiac (heart size, mediastinal contours), Diaphragm (costophrenic angles, free air), and Everything else (bones, soft tissues, tubes/lines). The silhouette sign occurs when two adjacent structures of the same density lose their border (e.g., right heart border loss = RML pathology; left heart border loss = lingula pathology).