1. An X-ray imaging system for displaying an image on a monitor, said system comprising an X-ray source, a collimator, and a masking unit comprising an adjustable shutter located between the X-ray source and the collimator, wherein the masking unit is configured to control the shutter so that part of said image is created at a higher refresh rate using a narrower X-ray beam than the rest of said image, said narrower beam defined by the shutter.