The information is usually collected by an X-ray beam that usually goes from front to back in the unwell patient. The resulting image contains a complex projection of intra-abdominal three-dimensional structures.

There is no ideal scheme for all observers and all situations. This particular version is suggested as a baseline that you may alter, depending on your own neurophysiology and your accumulating experience. In the discussion document on the abdomen, an entirely different order is used.

The Container

The viscera are confined by the walls of the trunk, the back muscles, the diaphragm and the extra-peritoneal pelvic viscera (supported by the pelvic diaphragm). The space can be regarded as two half cylinders alongside the spine and the main cylindrical space of the trunk. The asymmetry of information collection, the X-ray beam, will make the tangential margins more obvious.

The clear diaphragm silhouette is produced by the adjacent air-filled lower lobes. In both the abdomen and the chest, the diaphragm appears to have two parts. This is due to the tangental beam picking-up the density of the diaphragm as it passes forward from the rearward half cylinders of the para-spinal gutters. The lower anterior section is obscured by the heart and depressed by it. In the acute abdomen with free gas in the peritoneum, this anterior boundary may become visible and the serious observer will look for it.

The viewer will be aware of potential defects in the container, where gastric-hiatus, inguinal and femoral hernias may occur. Internal hernias and umbilical hernias should also be considered. Additional potential spaces for fluid or gas-containing abcess, lie behind the bladder (and uterus).

Retro-peritoneal Structures, outlined by fat.

Fat is visible because its main components of hydrogen and carbon have a lower atomic number than the tissues or water, as those who make their own butter can tell you.

The necessary perinephric fat makes the mobile kidneys almost always visible on the plain image. There is usually fat, picked-up by the tangential beam, at the round margin of the psoas muscles, but the psoas margin is not always visible in the normal. The tangential beam may also show the change in density produced by the pro-peritoneal fat at the inside lateral margin of the trunk.

The crura of the diaphragm are often visible, being picked-up by the tangential ray, adjacent to the lowest thoracic vertebra.

The pancreas cannot be seen, but deliberately looking for it will reveal displacement of neighbouring structures, abcess or calcifications that might be overlooked. Similarly the aorta is invisible, but displacement of other structures and linear calcification may reveal the unsuspected aneurysm.

Intra-peritoneal Structures, that may be outlined by fat.

The tangential beam will often show the inferior margins of the right lobe of liver. Liver size can often be infered from the positions of neighbouring gas-filled bowel.

The positions of gall-bladder and common bile duct are not visible, but thinking of their location may help detect contained gas or stones.

The spleen is not outlined, but seeking it will allow enlargement to be seen, especially in that patient who usually does not bring the whiskey bottle with him to the clinic.

The (usually unseen) falciform ligament is often surrounded by fat as it descends from the umbilicus into the liver. It is mentioned here as a reminder to look for this anterior feature, whenever there is suspected free gas in the abdomen.

Normal Gas 'lucency'

The individual gas-containing viscera can, potentially, be identified from cardia to anus. Note the invarying location of those sections that are partly retroperitoneal. Note also the site of the mesenteric attachments. Dilated (and thus lengthened) small-bowel remains constrained by its mesenteric attachment and its loops will have their centre of radius along the line of that attachment. The staircase of fluid levels in the erect view will also follow that line from duodeno-jejunal flexure to ileo-caecal valve, allowing the observer to guess the site of the small-bowel obstruction.

Departures from the normal pattern of gas may indicate pathology to the seasoned observer, for example the absence of haustra or contained faeces usually indicates ulcerative colitis.

Abnormal Gas 'lucency'

Fistulas (or the rare fulminant infection) may reveal other structures in the abdomen, particularly;
bile-ducts,
renal pelvis and ureters,
and bladder