Abdominal X-ray - System and anatomy
Bowel gas pattern

Any part of the bowel may be visible if it contains gas/air within the lumen. Gas/air is of low density and forms a natural contrast against surrounding denser soft tissues. It is often difficult to differentiate between normal small and large bowel, but this often becomes easier when the bowel is abnormally distended.

The upper limit of normal diameter of the bowel is generally accepted as 3cm for the small bowel, 6cm for the colon and 9cm for the caecum (3/6/9 rule).

Normal bowel sections are sometimes identified by the following features.

Stomach

The stomach may be visible if it contains gas/air. It is not visible if it is either completely empty, or fluid filled.

Small bowel (duodenum to terminal ileum)

Generally the small bowel lies centrally within the abdomen. The valvulae conniventes (also called plicae circulares) are thin, circular, folds of mucosa, some of which are circumferential and are seen on an X-ray to pass across the full width of the lumen.

Large bowel

The retroperitoneal structures of the colon (ascending colon, descending colon, and rectum) are relatively constant in position. These are often more readily identified than the transverse colon or sigmoid colon which are more variable in position. If visible, the caecum is often the widest segment. It too has a variable position, but is most often confined to the right iliac fossa.

The longitudinal muscles (taenia coli) and circular muscles of the colon form sacculations called haustra, which have characteristic radiographic appearance.

Another characteristic feature of large bowel is that it contains faeces. This has a mottled appearance due to its part gaseous content.