[Medical exposure from computed tomographic examination -a recommendation of dosimetry by film method-]

Already, the 30 year have passed since that the CT scanner was developed by Hunsfield. And now above 16,000 sets are at work. On the other hand, as the improvements of image reconstruction algorithm and variations os scan method, the medical exposure is increasing. For example, in the case of computed radiography, medical exposure is stability by using an auto exposure controller but in the case of CT, there is no unit and usually the exposure condition of CT examination are used that of recommended an exposure condition by maker. So the medical exposure greatly differs according to the operators or the kinds of CT scanner. In addition, there is the close connection between the x-ray intensity and the image quality. Especially, in the case of CT scanner there is no saturation of x-ray intensity like the optical density of film screen system because the digital radiographic systems have the wide dynamic range. So the increase in x-ray intensity makes a contribution to improvement of the image quality. But the increase in x-ray intensity makes a contribution to the increase in medical exposure. Therefore it is very important that the CT operators grasp the medical exposure per each CT examination and patient. There are many methods of medical exposure dosimetry on CT examination. But almost all the methods except the film method are impossible to get the medical exposure per each CT examination and patient. So in this paper the film method is recommended. Because the film method is low-priced, simple, easy and immediately shows the medical exposure per each CT examination and patient. Furthermore, it is confirmed that the film method is getting the correct medical exposure because that values gotten by the film method and ionization chamber are almost equivalent.