The increasing use of CT, especially in the last decade, has forced us to look at the issue of dose. A recent study from the National Academy of Sciences, examining a wealth of data accumulated over 10 years, announced that there is no threshold below which radiation is safe. Several months ago a governmental report indicated that radiation was a toxin. Whether you accept these two statements is probably beside the point. Despite disclaimers by several leaders in our specialty, the issue has now been raised to public consciousness. Increasing CT use will undoubtedly be accompanied by increasing surveillance of the possible ill effects of that use with respect to the induction of genetic damage, the instigation of cancers and the elaboration of vascular changes [1].

The debate has already been joined and the stakes are high. We can deny the issue at our peril; to claim that radiation has no ill effects will probably be reckless, to not change protocols and practice will be foolhardy, and to ignore the whole issue will open the door perhaps to later malpractice claims. So to be engaged in a discussion about dose means we have to talk the talk and learn the basic facts of the effects of radiation on human tissue and its relation to health. Most of you have been through this territory preparing for the physics part of the boards. You may have remembered everything from that time but even so things have changed. New units have gained popularity and new information has been generated about the deleterious effects of radiation along a range of doses.