Masaaki Akahane, Kuni Ohtomo

Department of Radiology, Graduate School of Medicine, University of Tokyo

　　In view of the increasing radiation burden from CT, we should make efforts to minimize the dose given to the patients. In this article, we discuss three essentials to achieve dose reduction : to be familiar with the dose quantities for CT, to be aware of the criterion for the upper limit of the dose, and to choose a low-dose scanner on renewal.
　　Volume CTDI（CT dose index）and DLP（dose-length product）are the essential dose measurements for CT examination. Original CTDI is the sum of the dose inside and outside the scanned slice. Volume CTDI is a weighted mixture of the central and peripheral CTDI with correction for the pitch. DLP is the product of volume CTDI and the scan length.
　　The reference dose values are proposed on the basis of the third quartile value of the distribution of doses in a wide-scale survey. The values can be regarded as the criterion of the upper limit of the dose.
　　Technical improvement in CT equipment such as automatic exposure control is necessary for unfailing dose management. If we choose a“low-dose”scanner on renewal, the manu-facturer will be encouraged to develop and implement further improvement.

Keywords：Computed tomography，CT dose index（CTDI），Automatic exposure control