Intra-peritoneal free gas in the child may appear slightly differently to free gas in the adult as erect chest radiographs are not usually performed. Signs to look for include:

• Lucency over the liver
• Extraluminal lucencies around the liver
• Extra-luminal triangles of free gas
• Riggler's sign - visualisation of both sides of the bowel wall
• Outlining of the falciform ligament
• The "football" sign - free gas seen as an anterior circular luncency projected over the mid abdomen
• Outlining of the lateral umbilical ligaments
• Outlining of the median umbilical ligament

On the following radiograph we can see several indicators of free gas: continuous diaphragm sign (short arrows, often seen in erect chest radiographs), Riggler's sign (small arrow heads) and extra-luminal free gas (large arrowheads) including a triangle of free gas surrounded by Riggler's sign (most medial large arrowhead). Also, a gastrostomy is shown - long arrow.

The second radiograph is unnecessary and should not have been performed. It does, however, demonstrate one of the possible methods of confirming suspected pneumoperitoneum (the horizontal ray left lateral decubitus radiograph). This is taken in the left lateral decubitus position (lying left side down) so that gas can be more easily seen around the liver. Note that this is not a lateral radiograph of the patient. Indeed, to say, as per convention, "left lateral decubitus" is to be tautologous and "left decubitus" with a description of beam (i.e. horizontal) would be preferrable.

An alternative method is the horizontal ray supine radiograph. N.B. I have dropped the word decubitus, but included the orientation of the rays as the alternative name of a "supine decubitus" radiograph would, if taken literally, mean a standard abdominal radiograph since supine decubitus is the position of the patient and is not solely a radiographic term.