objective: To propose a new set of Japanese diagnostic reference levels (DRLs) and achievable doses (ADs) for 2017 and to verify the usefulness of Japanese DRLs (DRLs 2015) for CT, by investigating changes in the volume CT dose index (CTDI vol ) from 2014 to 2017. Methods: Detailed information on the CT scan parameters used throughout Japan were obtained by questionnaire survey. The CTDI vol and dose-length product for the 11 commonest adult and 6 commonest paediatric CT examinations were surveyed and compared with 2014 data and DRLs 2015. results: Evaluations of adult head (helical), and abdomen and pelvis without contrast agent, paediatric chest without contrast agent, and abdomen and pelvis without contrast agent showed a slightly lower mean CTDI vol in 2017 than in 2014 (t-test, p < 0.05). The interquartile range of CTDI vol for all 2017 examinations was lower than in 2014. conclusions: This study verified the lower mean, 75th percentile, and interquartile range by investigating changes in the CTDI vol from 2014 to 2017. The DRLs 2015 contributed to CT radiation dose reduction. advances in knowledge: The widespread implementation of iterative reconstruction algorithms and low-tube voltage in CT scanners is likely to facilitate further reduction in the CT radiation dose used in Japan. Although radiological technologists may require further education on appropriate CTDI vol and DLP usage, the DRLs 2015 greatly contributed to the reduction of the CT radiation dose used in Japan.
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