When present,<unk>  can often be seen on projectional radiography, but small amounts are often missed, and CT scan is nowadays regarded as a criterion standard in the assessment of a<unk> . CT can visualize quantities as small as 5 cm³ of air or gas. Signs that can be seen on projectional radiography are shown below: The double wall sign marks the presence of air on both sides of the intestine. However, a false double wall sign can result from two loops of bowel being in contact with one another. The sign is named after Leo George Rigler. It is not the same as Rigler's triad. The football sign is when the abdomen appears as a large oval radiolucency reminiscent of an American football on a supine projectional radiograph. The football sign is most frequently seen in infants with spontaneous or iatrogenic gastric perforation causing<unk> . It is also seen in bowel obstruction with secondary perforation, as in Hirschprung disease, midgut volvulus, meconium ileus and intestinal atresia. Iatrogenic causes like endoscopic perforation may also give football sign. The Cupola sign is seen when air is accumulated under the central tendon of the diaphragm. Image:Pneumoperitoneum chest X-ray.Another<unk>  on chest X-ray. Image:Pneumoperitoneum lateral decubitus.Pneumoperitoneum seen on X-ray with the patient lying on his left side. File:Double wall sign annotated.Double wall sign. This is a secondary sign of<unk> . Patient is supine, and air within the abdomen and lumen of the bowel accentuate both sides of the bowel wall. File:UOTW 68 - Ultrasound of the Week 2.Ultrasound finding of<unk>  known as "peritoneal stripe sign" Differential diagnosis As differential diagnoses, a subphrenic abscess, bowel interposed between diaphragm and liver (Chilaiditi syndrome), and linear atelectasis at the base of the lungs can simulate free air under the diaphragm on a chest X-ray.
Pneumoperitoneum
Chilaiditi syndrome
Hemoperitoneum
Hepatic hydrothorax
Retroperitoneal bleeding
Acute abdomen
Pneumoretroperitoneum
Peritonitis