Radiology for Residents

Reading Abdominal Radiographs


Abdominal Radiographs: Normal

The standard abdominal radiograph taken is a supine projection: radiographs are passed from front to back (anteroposterior projection) of a patient lying down on back. In some circumstances an erect abdominal radiograph is requested: its advantage over a supine film is the visualization of air-fluid levels. A decubitus film (patient lying on side) is also of use in certain situations. An abdominal radiograph has a radiation dose equivalent to 50 posteroanterior chest radiographs 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 radiographs). 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 contrast between lung and ribs--black air against the white calcium containing bones. These differences are much less apparent on the ad as most structures are of similar density--mainly soft tissue.

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Figure 1. Normal

Figure 2. Rectal gas film

Figure 3. Valvulae conniventes


Technical Features

Ensures you are interpreting the correct film with the correct clinical information and it also may aid your interpretation. You would be a little concerned if you saw what appeared to be a calcified fibroid on an abdominal radiograph when holding the notes of Mr. John Brown. Next ask what type of abdominal radiograph is it: supine, erect, or decubitus? Unless specifically labeled the film is taken to be supine. The best way to appreciate normality is to look at as many films as possible, with an awareness of anatomy in mind (fig 1).

Intraluminal Gas

Begin by looking at the amount and distribution of gas in the bowels (intraluminal gas). There is considerable normal variation in distribution of bowel gas. On the erect abdominal radiograph, the gastric gas bubble in the left upper quadrant of the film is a normal finding. Gas is also normally seen within the large bowel, most notably the transverse colon and rectum (fig 2). Important characteristics of bowel loops to bear in mind are their size and distribution (where they are situated in relation to other structures). Normal small bowel should measure less than 3 cm in diameter, whereas normal colon should measure less than 5 cm in diameter. The diameter of the cecum may be greater, but if it is greater than 9 cm it is abnormal. Large bowel should lie at the periphery of the film, with small bowel distributed centrally. Small and large bowel can also be distinguished, most easily when dilated, by their different mucosal markings. Small bowel has valvulae conniventes that transverse the full width of the bowel; large bowel has haustra that cross only part of the bowel wall (figs 3 and 4). These features are important in the next part of this series, which considers abnormal intraluminal gas. Occasionally, fluid levels in the small bowel are a normal finding. Valvulae conniventes and haustra films Fecal matter in the bowel gives a "mottled" appearance (fig 5). This is seen as a mixture of grey densities representing a gas-liquid-solid mixture.

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Figure 4. Haustra films

Figure 5.

Figure 6. Gastric pseudotumour


It is important, as with any image, that the technical details of an abdominal radiograph are assessed. The date the film was taken and the name, age, and sex of the patient are all worth noting. Beware of gas in the portal vein, fine, lacy and peripheral, as this can look very similar to biliary air, central, porta hepatitis. Gas in the portal vein is always pathological and frequently fatal. It occurs in ischemic states, such as toxic megacolon, and it may be accompanied by gas within the bowel wall (intramural gas).

Calcification

Calcium is visible in a variety of structures, both normal and abnormal, and becomes more common with advancing age. However, review the following areas in particular for evidence of calcification: cartilage of ribs, blood vessels (chiefly the aortoiliac and splanchnic arteries), pancreas, kidneys, the right upper abdominal quadrant for gallbladder calculi, and the pelvis, which may contain a variety of calcified structures, most commonly phleboliths.

Places to look for abnormal extraluminal gas

  • Under the diaphragm
  • In the biliary system
  • Within the bowel wall

Soft tissues and bone

A review of the soft tissues entails evaluating the outlines of the major abdominal organs. Observing these structures is made easier by the "fatty" rim (properitoneal fat lines) surrounding them. In fact, the loss of these fat planes may indicate an ongoing pathological process, such as peritonitis. Look at the size and position of the liver and spleen. Look at the position and size of the kidneys, lateral to the midline in the region of the T12-L2 vertebrae (a useful way of identifying vertebrae: the lowest one to give off a rib is T12 and serves as a reference point). The renal outline is usually three to three and a half vertebral bodies in length. Also, look for the clear outline of the psoas muscle shadow(s). Finally, try to identify the outline of the bladder, seen more clearly if full, within the pelvis. The appearance of what looks like a soft tissue mass in the region of the stomach is more often than not actually a gastric pseudotumour. This is a normal finding on the supine film and represents gastric fluid lying within the fundus (fig 6). The assessment of bones entails evaluating the spine and pelvis for evidence of bony pathology. Osteoarthritis frequently affects the vertebral bodies, as well as the femoral and the acetabular components of the hip joint. Paget's disease may also be identified, commonly along the iliopectineal lines of the pelvis. Your bone survey should also check for fractures, especially subtle femoral neck fractures in elderly people. The spine and pelvis are also common locations for metastatic deposits. In the spine this is classically seen as "the absent pedicle."

Artifacts

You should be able to identify "man made" structures correctly. These may be iatrogenic (put there by health professionals), accidental (put there by the patient or other), or projectional (lying in front of or behind the abdomen but spuriously projected within it on the abdominal radiograph. Examples of iatrogenic structures would be surgical clips, an interuterine contraceptive device, renal or biliary stent, an endoluminal aortic stent, or inferior vena cava filter. Accidental findings include bullets or a per rectum object. Projectional findings include pajama buttons, coins in pockets, or body piercings (see part 6 of the series).

Key to densities in abdominal radiographs

  • Black--gas
  • Dark gray--fat
  • Gray--soft tissues
  • White--calcified structures
  • Intense white--metallic objects

Review Points

  • Technical specifics of the radiograph
  • Amount and distribution of gas
  • Extraluminal gas
  • Calcification
  • Soft tissue outlines and bony structures
  • Iatrogenic, accidental, and incidental objects

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