http://www.oncolink.upenn.edu/resour...&ss=222&id=959
... "Let’s take a look at the amount of radiation a particular organ receives with common radiologic studies. An x-ray at the dentist’s office results in the brain receiving 0.005 milligrays (or mGy- the measurement used to describe radiation doses). A routine chest x-ray results in the lung receiving 0.01 mGy, while a mammogram gives 3 mGy to the breast. An abdominal CT scan gives the stomach 10 mGy. In a baby, the exposure level is 20 mGy- 100-200 times the amount of radiation received with a typical x-ray.
But there is more. One study reported that almost all patients undergoing a CT of the abdomen or pelvis had more than one scan taken. Perhaps it was because the film quality was not good enough or the person moved during the scan, but whatever the reason, this results in multiple doses of radiation."