What is an oral cholecystogram (OCG)?
The oral cholecystogram, or OCG, is a radiologic procedure for diagnosing gallstones. An oral cholecystogram is in contrast to an intravenous cholangiogram (IVC).
How is an oral cholecystogram done?
For an OCG, the patient takes iodine-containing tablets by mouth for one night or two nights in a row. The iodine is 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 in an OCG because it is dense and radioopague (stops x-rays). 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. Therefore, any gallstones lodged in the ducts will go undetected on OCG.
Failure to visualize the gallbladder on an OCG may occur for one of two reasons. First, a gallstone may have obstructed the cystic duct and prevented the iodine-containing bile from entering the gallbladder. Second, inflammation of the gallbladder -- usually due to gallstones -- may have interfered with the gallbladder's ability to concentrate bile and iodine. Whichever the cause, failure to visualize the gallbladder on an OCG strongly suggests a diseased gallbladder.
What are the risks of an oral cholecystogram?
The risks of an oral cholecystogram are few. One, of course, is the radiation. However, in an OCG there is a relatively small amount of radiation from the x-rays.
Another risk involves the iodine. Some people are allergic to iodine. Fortunately, most people who are allergic know that they are, and therefore do not undergo the test.
When is an oral cholecystogram useful?
The OCG is an excellent procedure for diagnosing gallstones. It finds gallstones 95% of the time. However, the OCG has tended to be replaced by ultrasonography because ultrasonography is slightly better at diagnosing gallstones and can be done immediately without waiting one or two days for the OCG's iodine to be absorbed, excreted, and concentrated. The OCG also cannot give information about the presence of non-gallstone related diseases, which ultrasonography is sometimes able to do. A limitation of the OCG is that it does not work well when there is more than a minimal amount of jaundice. Fortunately, most people with gallstones are not jaundiced.
As would be expected, ultrasonography sometimes finds gallstones that are missed by the OCG. Less frequently, the OCG finds gallstones that are missed by ultrasonography. For this reason, if gallstones are strongly suspected but ultrasonography does not show them, it is reasonable to consider doing an OCG. So OCGs are still done, and for good reason.