An apparatus and a method are provided for controlling collimation operation of a collimator in an x-ray imager during an intervention carried out by an operator. A first medical device is made to progress through a patient in an exploratory phase of the intervention. A path is recorded in a scout image formed from footprints of the first device as recorded in a sequence of projection images acquired by imager during the exploratory phase. Apparatus uses the path in the scout image to control collimation of the x-ray beam during a subsequent delivery phase where a second medical is made to progress through patient, said second device following the first device substantially along the same path.