For the purpose of determining a radiation dose without excess or insufficiency with respect to an allowed value of image noise in performing a helical scan by a CT apparatus comprising a multi-detector, a method comprises the steps of: selecting an image thickness of an X-ray tomographic image to be produced by a helical scan by an X-ray CT apparatus comprising a multi-detector (Step ST1); provisionally determining an X-ray dose in obtaining the X-ray tomographic image having the image thickness by single-slice CT using a single-slice CT radiation dose determining algorithm (Step ST2); selecting a scan protocol (Step ST3); reading a dose correction factor from a dose correction factor table that matches the selected image thickness (Step ST4); correcting the X-ray dose by the dose correction factor (Step ST5); determining tomographic imaging scan conditions by specifying at least one of the tube current and the emission time (Step ST6); and performing the helical scan and displaying an X-ray tomographic image (Step ST7). Consequently, a disadvantage that a subject is exposed more than necessary can be prevented.
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