This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal Radiology, Subject name: Radiology
Air on either side of the bowel wall on a radiograph is called: 
Select and explain. 
A. Air under diaphragm sign, B. Cupola sign, C. Doges cap sign, D. Rigler sign
D: Rigler sign
Answer: D: Rigler sign 
Explanation: The Rigler sign, also known as the double-wall sign, is a sign of pneumoperitoneum seen on an abdominal radiograph when gas is outlining both sides of the bowel wall, i.e. gas within the bowel's lumen and gas within the peritoneal cavity. It is seen with large amounts of pneumoperitoneum (>1000 mL).