When performing a fly-by or helical CT scan of a subject, radiation dose is limited by positioning a dynamic collimator (142) between the subject and an X-ray source (112). The collimator moves axially with the X-ray source (112) along a volume of interest (VOI) (122) in the subject and gradually opens, such that a narrow portion of the cone beam of X-rays is permitted to pass through the collimator (142) at ends of the VOI (122) and a wider full cone beam is emitted at central portions of the VOI (122). In this manner, tissue surrounding the VOI (122) is not needlessly exposed to X-rays, as would be the case if a full-width cone beam were used for the entire scan of the VOI (122).