sail sign of the chest sail sign of the chest on a chest x-ray, the sail sign is a radiologic sign that suggests left lower lobe collapse. in children, however, a sail sign could be normal, reflecting the shadow of the thymus. sail sign of the chestdifferential diagnosisleft lower lobe collapse the thymic sail sign or spinnaker-sail sign is due to elevation of the thymic lobes in the setting of pneumomediastinum.