A sentinel loop is a sign seen on a radiograph that indicates localized ileus from nearby inflammation. Simply put, it is the dilatation of a segment of small intestine. to be differentiated from colonic cutoff sign which is a dilatation of a segment of large bowel.
An isolated distended loop of bowel is seen near the site of injured viscus or inflamed organ. This loop is called a "sentinel loop." It arises from the body's efforts to localize traumatic or inflammatory lesions. The local distention of that intestinal loop is due to local paralysis and accumulation of gas in the intestinal loop.
In acute pancreatitis, the sentinel loop is usually seen in left hypochondrium, while in acute cholecystitis, it is seen in the right hypochondrium. In acute appendicitis, the sentinel loop is seen in right iliac fossa.
http://www.hawaii.edu/medicine/pediatrics/pemxray/v3c18m.jpg