The supine chest x-ray shows a deep sulcus sign on the left, which is a radiological sign seen in supine patients with pneumothorax.

The deep sulcus sign represents lucency of the lateral costophrenic angle extending towards the hypochondrium. In a supine patient, air in the pleural space tends to accumulate anteriorly and basally within the non-dependent region of the pleural space, causing deepening of the lateral costophrenic angle.

Here is a coronal CT chest slice from the same patient showing the deep sulcus clearly:

CT scan of the chest also showed multiple left-sided rib fractures and a comminuted left scapular fracture apart from the left pneumothorax.