1. A dose evaluating method comprising: collecting fixed view projection data using a scout scan performed without rotating an X-ray tube and an X-ray detector about a body axis of a subject; estimating a dose for use in a scan performed by rotating at least one of the X-ray tube and the X-ray detector about the body axis of the subject, the dose estimated based on the fixed view projection data and based on at least one of a region of the subject imaged during the scan and an imaging purpose of the scan; displaying the estimated dose as a maximum dose before the scan; selecting a single constant dose value to be used during the scan based on the maximum dose; performing the scan using the single constant dose value throughout the scan to produce projection data; and generating an image based on the projection data, the image having image noise one of less than a predetermined value and equal to the predetermined value.