Small intestinal loops are usually filled with gas and easily identified. Gas in the GI tract in patient with obstruction comes mainly from aerophagia. The diagnosis of intestinal obstruction may be very challenging on survey radiographs. Care should be taken to obtain a complete abdominal study of good technical quality. This segment is sometimes called a sentinel loop.
Colon is normally air filled and of larger diameter than small intestine.
When identifying a large air filled bowel loop, one must determine if it represents a segment of dilated small intestine or a segment of normal colon. Respiratory distress can cause severe aerophagia leading to a pattern that can be confused with intestinal obstruction. Chronic incomplete intestinal obstruction is usually associated with slow growing intestinal tumor and may show a specific pattern characterized by accumulation of feces-like granular material in the small intestine oral to the obstruction. On survey radiographs, the abnormal small bowel loop looks like colon filled with feces. Pneumocolon may be indicated to positively identify the colon.