Current state of radiation exposure in xenon CT cerebral blood flow examination in Japan and decreasing of radiation exposure

Xenon CT cerebral blood flow (CBF) examination has much radiation exposure because of frequent scanning of the same slice. First, we did questionnaire investigation in Japanese 202 institutions in three months since December, 2004. The questionnaire result showed that the number of scan slices was 2.6±1.3 (mean±Sd). The number of scans per one slice was 9.3±1.7. The x-ray tube voltage was 120 kV in most institutions. The x-ray tube current was 347±128 m As. We got an answer from a 54 institution about weighted CT dose index (CTDIw) value. The radiation exposure in these institutions was 1,108.3+/-874.4 mGy· cm. Second, we studied the influence that decrease of scan frequency gave to radiation exposure and cerebral blood flow value. In a 4 slices scan with a CT device of CTDIw 39.6 mGy, radiation exposure decreased by 634 mGy when we changed scan frequency from seven times to three times. The decrease of scan frequency resulted in cerebral blood flow increase of 3-5 ml/100 mg/min. In conclusion, among Japanese institutions, number of scan slices and scan frequency per one slice were various. Making of a guideline and reconsideration of the scan conditions are necessary for reduction of radiation exposure. (author)