The reasoning
| Projection | Technique | Indication / effect |
|---|---|---|
| PA erect | beam posterior→anterior, anterior chest on detector, centred at T7 (inferior angle of scapula), SID ~180 cm, full inspiration, scapulae protracted | standard; minimal cardiac magnification; true CTR |
| AP (supine/semi‑erect) | portable; short SID | ill patients; cardiac magnification, scapulae overlap lungs, effusion layers posteriorly, air rises anteriorly |
| Lateral | left side to detector by convention | localisation; retrosternal and retrocardiac spaces; posterior costophrenic recess (shows ~75 mL) |
| Lateral decubitus | horizontal beam, patient on side | smallest detectable effusion on radiograph (~5–25 mL); mobile vs loculated fluid (affected side down); pneumothorax (affected side up) |
| Lordotic / apical | beam angled cephalad / patient leans back | apices free of clavicles; RML collapse |
| Expiratory | at end expiration | air trapping (FB aspiration), diaphragmatic paralysis; small pneumothorax (no longer routine) |
| Oblique | rotated | rib fractures, pleural plaques (largely replaced by CT) |