Barium has a high atomic number and absorbs X-rays extremely well. A ‘barium meal’ consisting of an insoluble barium salt such as barium sulfate, BaSO4, is given to patients to swallow in the form of a milky-looking drink, and its progress through the digestive system is followed with X-rays. This is typically used to visualise the structures of the upper gastro-intestinal tract. For the lower parts of the intestines, including the bowel, a barium enema is given instead.
Why is it desirable for this contrast agent to be insoluble? (The solubility product of BaSO4 is )
The very low solubility product of BaSO4 means that this is not absorbed in the body but is simply excreted with no danger. (In fact, soluble salts of barium are highly poisonous.)
You may recall that the units of solubility product will differ depending on the expression for the sparingly soluble salt concerned. Account for the units shown above for barium sulfate.
The equilibrium for barium sulfate is:
So the solubility product is given by
and in this case the units will be:
Abnormalities such as ulcers in the stomach wall and abnormal growths can be picked up using a barium meal.
Figure 4 shows an X-ray of the large intestine of a patient using this method. In this example, the contrast of the image has been reversed to see the intestines better and hence allow the medical practitioner to make a diagnosis.
Contrast agents are also used to enhance the image in organs such as the kidneys, liver and bladder, as well as in bronchography (imaging of the lower respiratory tract).