Data from two sources provide an estimate that in 1983, 5-5.5 million computed tomographic (CT) procedures were performed in the U.S. Of those, 63% were head scans, 27% body, and 10% spine. Ten percent or fewer were high-resolution studies, primarily spine CT. The surface dose varies widely but ranges between 1 and 7 rad (10-70 mGy) for most examinations. It is estimated that CT scanning contributes 7%-8% of the adult active bone marrow dose that is received from diagnostic radiology.