The chest x-ray shows evidence of a pneumoperitoneum. There is a thin sliver of air under the right hemidiaphragm. The radiological sign that can be seen is the ‘Cupola’ sign; this is an inverted semilunar line with an upwards convexity that is located just below the heart. This is due to the collection of air underneath the central tendon of the diaphragm (i.e. the cupola).
However, the Cupola sign is not a common radiographic presentation of pneumoperitoneum. The cause of the pneumoperitoneum in this patient was a sigmoid diverticular perforation.
Further reading here.