The oral cholecystogram (OCG) is a radiologic procedure for diagnosing gallstones and it is in contrast to an intravenous cholangiogram (IVC). It finds gallstones 95% of the time.
Ultrasonography is tended to replace OCG because it can be done immediately without waiting one or two days for the OCG's iodine to be absorbed, excreted, and concentrated and it can give information about the presence of non-gallstone-related diseases, which OCG cannot.
The OCG also does not work well when there is more than a minimal amount of jaundice but fortunately, most people with gallstones are not jaundiced.
Ultrasonography sometimes finds gallstones that are missed by the OCG but OCG can also find gallstones that are missed by ultrasonography.
Before the OCG procedure, the patient takes iodine-containing tablets by mouth for one night or two nights in a row which are absorbed from the intestine into the bloodstream, removed from the blood by the liver, and excreted by the liver into the bile. The iodine, together with the bile, is highly concentrated in the gallbladder.
Iodine is used because it is dense and radioopaque (stops x-rays) and it outlines the gallstones that are radiolucent (x-rays pass through them) and that are usually invisible on x-ray.
The bile ducts themselves cannot be seen on the x-ray in an OCG because the iodine is not concentrated in the ducts so any gallstones lodged in the ducts will go undetected on OCG.
There are few risks of an OCG and one is the radiation, but there is a relatively small amount of radiation from the x-rays during OCG and the second one involves the iodine because some people are allergic to iodine but most people who are allergic know that they are, and therefore do not undergo the test.