The erect AP chest x-ray shows a sliver of air under the medial aspect of the right hemi-diaphragm.

The patient underwent a CT scan of the abdomen, which confirmed the presence of pneumoperitoneum secondary to a perforated ileum.

Prior to obtaining an erect chest x-ray, the patient should remain in the erect position for 10 minutes to allow for any free gas to rise to the diaphragm.

In a unwell patient who cannot be moved out of resus, a left lateral decubitus abdominal x-ray can help to identify gas over the liver.

Reference: Grainger and Allison’s Diagnostic Radiology: A Textbook of Medical Imaging.