Pneumoperitoneum
From medscape
Pneumoperitoneum (air within the peritoneal cavity) most commonly results from perforation of abdominal viscus. On upright chest radiographs, dark crescents of air (blue arrows) will separate the liver, spleen and intestines from diafagm. Since air will acumulat from uppermost portion of abdominal cavity, patients should be kept upright at least 5 minutes before the image is taken, to ensure the adequate air migration. Sometimes air inside the bowel outlines the intestinal wall - double walk or Ringle sign ( red arrow)
