Is a deep sulcus sign present in these radiographs?
No (as probably no significant amount of air has collected inferolaterally)
What should we have in our mind when reviewing supine chest radiographs?
The supine projections underestimate the size and presence of pneumothorax.
Note the visible visceral pleural edge and a relatively increased transparency of the involved left hemithorax on the supine AP radiograph. This occurs as in the supine position gravity causes the lung to fall dorsally allowing the air to collect anteromedially, laterally and basally.
In the erect projection the large pneumothorax is far more evident since the lung is positioned more inferiorly. Thus, air is seen at the apex of the chest showing the familiar appearance of the visceral pleural line parallel to but separated from the chest wall.