Abdominal x rays made easy: abnormal intraluminal gas
Got a blockage in your learning? Let Ian Bickle and Barry Kelly help by explaining bowel obstruction and other causes of abnormal intestinal gas, in the second part of our series on abdominal radiographs
On an abdominal radiograph, as with all plain film images, four densities can be seen--white, grey, slightly darker grey, and black--representing bone, soft tissue, fat, and air. Metallic objects are seen as intense bright white. The abdominal radiograph is a representation of the abdominal viscera and bowel: the presence of gas in most instances is normal. Several medical and surgical conditions, however, are recognisable by an abnormal amount, distribution, or location of air on the radiograph. Abnormal gas can be (a) intraluminal, in the stomach, duodenum, and intestine, or (b) extraluminal--that is, elsewhere.
Most intraluminal gas is in the large intestine, which has the greatest luminal diameter of the intestinal tract. A diameter of more than 5 cm suggests a large bowel obstruction and would be considered abnormal (except in the caecum). As the intestine is a large long tube, any obstruction, either from within or by external compression, prevents