Why is barium used instead of bismuth as a contrast in gastrointestinal xrays?
Is it less expensive?


Yes, but not so much that it would be favored, i.e. you don't worry about saving \$5 when your doing a \$5000 procedure.


  Is there a particular property of barium that makes it a better contrast?


No both absorb x-rays as a result of their high atomic numbers, in fact bismuth has more electrons per atom and thus is a better contrast agent..


  Bismuth is a heavy metal that has a relatively low toxicity in comparison to barium.


Yes but this is for the soluble metal ion. Barium sulfate is inert under most conditions, importantly the acidic ones in the digestive tract. Because the sulfate binds the barium ions to the solid and thus barium cannot enter the blood stream to cause a physiological response. Contrast this to Bismuth (no pun intended) which is non-toxic but typically soluble in acid and if not soluble, the bismuth is at least leachable. Now typically bismuth ions are not an issue as they will harmlessly be excreted, but there is a potential it could interact with a patient or the drugs of a patient in a negative or uncontrolled way. Since doctors don't like to introduce variables when making diagnosis's nor do the like surprises, especially when it's caused by them, the fully inert barium sulfate is the favorite for radio contrast.