Ascites

Abnormal accumulation of fluid in the peritoneal cavity of the abdomen. Usually caused by chronic conditions such as cirrhosis of the liver.

Image: Bowel loops displaced medially by ascitic fluid lying between properitoneal fat line and colon. Inferior angles of liver and spleen obscured. Fluid attenuation superior to bladder 

Volvulus

The twisting of a loop of the intestine, which creates an obstruction. Volvulus may require surgery for correction.

Image: The x-rays taken in the ED show a distended loop of large bowel with a liquid-air level between the right diaphragm and the liver. Furthermore they demonstrate some distended loops of the distal small bowel. 

Pneumoperitoneum

Free air or gas in the peritoneal cavity. When caused by perforation of a gas-containing viscus, surgery is needed. Small amounts of residual air can be seen 2-3 weeks after.

Image: In a normal x-ray, only the luminal surface (blue arrows) should be visible outlined by gas. The serosal surface (orange) should not be visible as it is normally in contact with other intra-abdominal content of similar density (other loops of bowel, omentum, fluid). In this case, gas abuts the serosal surface rendering it visible 

Intussusception

The telescoping of a bowel section into another bowel section, creating an obstruction. Most common in the distal of the small intestine and in children.

Image: Dilated small bowel loops (supine view) with multiple large air-fluid levels (erect view) respectively, suggestive of intestinal obstruction. 

Fibrous Adhesions

The most common cause of mechanically based obstruction. A fibrous band of tissue interrelates with the intestine, creating a blockage. 

Image: There is mild dilatation of the small bowel seen at the mid-abdomen with associated air-fluid levels denoting underlying element of bowel obstruction. 

Image: No evidence of pneumoperitoneum. There are several central loops of small bowel which are moderately dilated and demonstrate air-fluid levels on the erect projection suggesting a small bowel obstruction. There is no gas or feces seen in the large bowel suggesting it is collapsed distal to the point of obstruction. No bowel wall thickening appreciated. The hernial orifices are not visualized.