A 30-year-old trauma patient with suspected pneumothorax requires a prompt and reliable imaging modality to confirm the diagnosis and guide management. Chest X-ray (CXR) is the most accessible and rapid initial diagnostic tool that can detect air in the pleural space, indicating pneumothorax. It helps assess the size and extent of the pneumothorax, which is critical for surgical decisions such as tube thoracostomy. Ultrasound can be useful but is operator-dependent and less available in some settings; abdominal ultrasound and CT brain do not evaluate the thoracic cavity, and ECG is unrelated to pneumothorax diagnosis. Therefore, Chest X-ray is the first-line imaging study in trauma patients with suspected pneumothorax.

Reference: Tintinalli's Emergency Medicine, Judith E. Tintinalli, 9th Edition.