A scanner system comprises: a radiation generator arranged to generate radiation to irradiate an object, the radiation generator comprising a radiation source arranged to produce radiation and a filter arranged to provide variable filtering of the radiation from the source; a detector structure arranged to detect the radiation after it has interacted with the object and generate a sequence of detector data sets as the object is moved relative to the generator, and a processing system arranged to process each of the detector data sets thereby to generate a control output arranged to control the radiation generator so as to vary the filtering, thereby to vary the radiation output by the radiation generator as the object is scanned.