Abdominal x rays made easy: normal radiographs

The abdominal radiograph is one of the most commonly requested images, and all medical students should have a knowledge of common radiological interpretations. This article covers the radiology of normal findings. Subsequent parts of the series will cover abnormal intraluminal gas, abnormal extraluminal gas, calcification, bone and soft tissue abnormalities, and iatrogenic, accidental, and incidental objects.

The standard abdominal radiograph (AXR) taken is a supine projection: x rays are passed from front to back (anteroposterior projection) of a patient lying down on his or her back. In some circumstances an erect AXR is requested: its advantage over a supine film is the visualisation of air-fluid levels. A decubitus film (patient lying on his or her side) is also of use in certain situations.

Although an AXR is a plain radiograph, it has a radiation dose equivalent to 50 posteroanterior chest x rays or six months of standard background radiation.

As with any plain radiograph, only five main densities are seen, four of which are natural: black for gas, white for calcified structures, grey representing a host of soft tissue with a slightly darker grey for fat (as it absorbs slightly fewer x rays). Metallic objects are seen as an intense bright white. The clarity of outlines of structures depends, therefore, on the differences between these densities. On the chest radiograph, this is easily shown by the …