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Objective: The aim of the study was to investigate the effective dose of patient caused by PET/CT to provide data basis for decreasing the radiation dose.Methods: The effective dose was the sum of the dose caused by CT and PET.The dose caused by PET was calculated by the radioactivity of the radiopharmaceutical injected.The dose caused by CT was determined by acquisition for the anthropomorphic phantom(Model RS-550).PET/CT system of GE Discovery ST-16 was used to simulate clinical PET/CT situation for the anthropomorphic phantom.CT acquisition conditions were as follows: tube voltage 120kV,tube current 30-250mA,pitch 0.938,1.375 and 1.75.Scan range was divided into body scan and head-neck scan to simulate clinical PET/CT.The dose-length product(DLP)was recorded and the effective dose(EDCT)was calculated with EDCT =k·DLP.Results: 1)The effective dose caused by PET was 3.16-8.96 mSv for a patient with 45-85kg weight with injection dose 3.70-5.55MBq/kg(0.08-0.15mCi/kg)of 18F-FDG,increased linearly with weight and injection dose and independent of the PET acquisition conditions.2)The effective dose caused by CT increases linearly with increasing tube current when the pitch was fixed.However,when the tube current fixed,the effective dose reduce with increasing pitch.Under the different acquisition condition,the effective dose was 1.62-27.68 mSv.Conclusions: There is no big variation for effective dose by PET for a certain patient,however there is much difference for the effective dose caused by CT under the different acquisition conditions.It shows that the radiation dose caused by CT in PET/CT could be effectively reduced by appropriate choice of acquisition conditions according to different clinical needs.