The detection of intraperitoneal free air in the setting of trauma can often be very challenging. However, such cases usually need urgent surgical intervention. Plain radiographs being an initial, cheap investigation, still play a vital role in guiding further investigations and management. A study compared the sensitivity of supine radiographs with erect chest and decubitus abdominal radiographs in the detection of pneumoperitoneum. They found the positive predictive value to be 80.4% on supine KUB (Kidney, ureter, and bladder study), 78.7% on supine chest radiograph, 85.1% on the erect radiograph, and 98.0% on left decubitus abdominal radiograph. 
It is often not possible or feasible to perform an erect chest radiograph in trauma patients. Radiologists should be familiar with the signs of pneumoperitoneum on supine radiographs such as Rigler sign (double-wall sign), falciform ligament sign, ligament teres sign, cupola sign (radiolucency seen below the heart), football sign (large oval radiolucency over the central abdomen) and the hepatic edge sign. The named signs in the pediatric radiographs include inverted V-sign (air outlining the lateral umbilical ligaments) and the urachus sign.  Sometimes only a subtle right subphrenic radiolucency may be visible, such as in the above case.
Air pockets localized adjacent to the bowel wall constitute a direct sign of perforation and can help predict the perforation site.  Acute mesenteric ischemia is a secondary sign of bowel perforation and has a poor prognosis if not treated early. The complications include bowel necrosis, gangrene, and perforation. Multidetector CT with contrast including arterial and portal venous phase is the recommended protocol. A split bolus technique allows a single-phase acquisition by dividing the contrast dose with a timed delay.  It has the advantage of lower radiation doses. Specific signs of bowel ischemia include lack of bowel wall enhancement, pneumatosis intestinalis, gas in the portal vein, and pneumoperitoneum.