1. A computed tomography imaging machine comprising: a radiation source ( 16 , 18 ) providing a radiation beam divisible into regions ( 30 ) being individually controllable in intensity; a radiation detector ( 28 ) for detecting the radiation from the radiation beam to produce attenuation signals; a patient support ( 22 ) supporting a patient between the radiation source and radiation detector; and a controller ( 40 ) holding a stored model ( 53 ) of the patient and controlling the intensities of the radiation beam in the different regions based on a contribution ( 60 ) by the radiation of the regions to a quality of a tomographic image as determined from the stored model.